[Galaxy S II Q&A, Help & Troubleshooting] blue whale:
For kitkat, there are CWMs for android 4.1.2 which are compatible with it and some are not.

get one compatible here https://xdaforums.com/galaxy-s2/general/cwm-kit-kat-compatible-t2546597

It is not kitkat, but the old stock roms are here https://www.sammobile.com/firmwares/galaxy-s-/GT-I9100T/ and not free. I think the i9100 and i9100T are the same, but the T is for australians.

what you can do is google the names of these roms and try to find a valid link in 2019.

then you need the australian modem which were here http://veyka.co.uk/sgs2/radios/ but I have a local copy and can give you them

Here is the way to install stock 4.1.2

https://xdaforums.com/galaxy-s2/help/i9100-useful-fixes-lineageos-14-1-t3575340

and root

https://xdaforums.com/galaxy-s2/general/how-to-root-stock-recovery-t1994544

and install twrp 3 with 4.1.2 ????

https://xdaforums.com/showpost.php?p=77492624&postcount=106

when ROm 41.1.2 is installed, you either get only twrp 2.8.1 or some philz_touch_6.48_CWM recoveyr, or you go with an ISOREC kernel [like DORIMANX here https://xdaforums.com/galaxy-s2/development-derivatives/kernel-dorimanx-jb-4-1-2-t3106241] +ISOREC twrp ...

https://xdaforums.com/galaxy-s2/help/how-to-install-twrp-straight-samsungs-t3528581

then you stay on 4.1.2., or kitkat, or LOS14.1

https://xdaforums.com/showpost.php?p=74611459&postcount=4

https://xdaforums.com/galaxy-s2/help/help-to-install-twrp-3-1-0-0-lineageos-t3706803

---------- Post added at 10:29 AM ---------- Previous post was at 09:58 AM ----------

here are all the radio modems for the samsung s2 i9100 from the dead site http://veyka.co.uk/sgs2/radios/

I9100_radio_BOKJ1_CWM.zip

https://www95.zippyshare.com/v/JYRE1EWO/file.html

I9100_radio_BULP7_CWM.zip

https://www95.zippyshare.com/v/2h0w965q/file.html

I9100_radio_BVKJ4_CWM.zip

https://www95.zippyshare.com/v/jmUYDjS3/file.html

I9100_radio_BVKL1_CWM.zip

https://www95.zippyshare.com/v/7z4fME64/file.html

I9100_radio_BVLP7_CWM.zip

https://www95.zippyshare.com/v/2R64mvGV/file.html

I9100_radio_BVLPB_CWM.zip

https://www95.zippyshare.com/v/luY9wgDn/file.html

I9100_radio_DDLP8_CWM.zip

https://www95.zippyshare.com/v/zQB9Y4vb/file.html

I9100_radio_DDLPA_CWM.zip

https://www95.zippyshare.com/v/qrcwkFMW/file.html

I9100_radio_DULP3_CWM.zip

https://www95.zippyshare.com/v/l9igMJxZ/file.html

I9100_radio_DXKL1_CWM.zip

https://www95.zippyshare.com/v/HUqDgEEI/file.html

I9100_radio_DXKPA_CWM.zip

https://www95.zippyshare.com/v/rWaIqAF8/file.html

I9100_radio_DXLP7_CWM.zip

https://www95.zippyshare.com/v/0ZWMyylZ/file.html

I9100_radio_DXLP9_CWM.zip

https://www95.zippyshare.com/v/saKumAuj/file.html

I9100_radio_NEKF2_CWM.zip

https://www95.zippyshare.com/v/lturTv7K/file.html

I9100_radio_NEKI2_CWM.zip

https://www95.zippyshare.com/v/jKp1nd2B/file.html

I9100_radio_NELP2_CWM.zip

https://www95.zippyshare.com/v/UjNtpslC/file.html

I9100_radio_NELP4_CWM.zip

https://www95.zippyshare.com/v/J6QM4VN8/file.html

I9100_radio_Template_CWM.zip

https://www95.zippyshare.com/v/xHW45Iw5/file.html

I9100_radio_UHLPE_CWM.zip

https://www95.zippyshare.com/v/r1JgmkNr/file.html

I9100_radio_UHLPG_CWM.zip

https://www95.zippyshare.com/v/JGm7E70i/file.html

I9100_radio_WXKI3_CWM.zip

https://www95.zippyshare.com/v/3xd4AnRk/file.html

I9100_radio_XXKE4_CWM.zip

https://www95.zippyshare.com/v/DYGnpxQw/file.html

I9100_radio_XXKE7_CWM.zip

https://www95.zippyshare.com/v/jw5tUQ0V/file.html

I9100_radio_XXKF2_CWM.zip

https://www95.zippyshare.com/v/70zCbLid/file.html

I9100_radio_XXKG1_CWM.zip

https://www95.zippyshare.com/v/JwYqlP2k/file.html

I9100_radio_XXKG2_CWM.zip

https://www95.zippyshare.com/v/eGZ7uzZ9/file.html

I9100_radio_XXKG3_CWM.zip

https://www95.zippyshare.com/v/MHy1Kgj4/file.html

I9100_radio_XXKG5_CWM.zip

https://www95.zippyshare.com/v/9olib4qE/file.html

I9100_radio_XXKG6_CWM.zip

https://www95.zippyshare.com/v/DZBD5Y9t/file.html

I9100_radio_XXKH1_CWM.zip

https://www95.zippyshare.com/v/UaeYKv4f/file.html

I9100_radio_XXKH3_CWM.zip

https://www95.zippyshare.com/v/QLMRENjF/file.html

I9100_radio_XXKI1_CWM.zip

https://www95.zippyshare.com/v/15YRYzwD/file.html

I9100_radio_XXKI3_CWM.zip

https://www95.zippyshare.com/v/0MzJvhhm/file.html

I9100_radio_XXKI4_CWM.zip

https://www95.zippyshare.com/v/SgKRJDFA/file.html

I9100_radio_XXKL1_CWM.zip

https://www95.zippyshare.com/v/sqzqFBpw/file.html

I9100_radio_XXKP1_CWM.zip

https://www95.zippyshare.com/v/EDTDTD8y/file.html

I9100_radio_XXKP8_CWM.zip

https://www95.zippyshare.com/v/1vTt1Xc1/file.html

I9100_radio_XXKPA_CWM.zip

https://www95.zippyshare.com/v/ObqQDF5I/file.html

I9100_radio_XXLP1_CWM.zip

https://www95.zippyshare.com/v/6MZMtj2F/file.html

I9100_radio_XXLP6_CWM.zip

https://www95.zippyshare.com/v/pItYWANM/file.html

I9100_radio_XXLPB_CWM.zip

https://www95.zippyshare.com/v/rpbu4FZr/file.html

I9100_radio_XXLPH_CWM.zip

https://www95.zippyshare.com/v/rzhJGNZx/file.html

I9100_radio_XXLPJ_CWM.zip

https://www95.zippyshare.com/v/u8oHnVHg/file.html

I9100_radio_XXLPQ_CWM.zip

https://www95.zippyshare.com/v/1fap9MGe/file.html

I9100_radio_XXLPS_CWM.zip

https://www95.zippyshare.com/v/1EtWo3CC/file.html

I9100_radio_XXLPW_CWM.zip

https://www95.zippyshare.com/v/FuwvmHcj/file.html

I9100_radio_XXLPX_CWM.zip

https://www95.zippyshare.com/v/F2Kj00bw/file.html

I9100_radio_XXLQ5_CWM.zip

https://www95.zippyshare.com/v/K20REhVZ/file.html

I9100_radio_XXLQ6_CWM.zip

https://www95.zippyshare.com/v/AMPERkFZ/file.html

I9100_radio_XXLS6_CWM.zip

https://www95.zippyshare.com/v/X4Q0ndXE/file.html

I9100_radio_XXLS8_CWM.zip

https://www95.zippyshare.com/v/9PPhcVcy/file.html

I9100_radio_ZCLPK_CWM.zip

https://www95.zippyshare.com/v/sdklBFNn/file.html

I9100_radio_ZSLPF_CWM.zip

https://www95.zippyshare.com/v/8Ymb7Mx5/file.html

I9100_radio_ZSLPG_CWM.zip

https://www95.zippyshare.com/v/aUmVBqnz/file.html

I9100M_radio_UGLD3_CWM.zip

https://www95.zippyshare.com/v/1vY4HBqK/file.html

I9100T_radio_DCKF1_CWM.zip

https://www95.zippyshare.com/v/534doEBM/file.html

I9100T_radio_DCKL1_CWM.zip

https://www95.zippyshare.com/v/Qxwc93zj/file.html

I9100T_radio_DCLP4_CWM.zip

https://www95.zippyshare.com/v/JLeqGuAe/file.html

I9100T_radio_DOKG2_CWM.zip

https://www95.zippyshare.com/v/XKIszPhR/file.html

I9100T_radio_DOLP4_CWM.zip

https://www95.zippyshare.com/v/IMntuu0y/file.html

I9100T_radio_DUKF4_CWM.zip

https://www95.zippyshare.com/v/DMBEjEN3/file.html

I9100T_radio_DVKF2_CWM.zip

https://www95.zippyshare.com/v/IXd3GVPQ/file.html

I9100T_radio_DVLP3_CWM.zip

https://www95.zippyshare.com/v/2xp25wcQ/file.html

I9100T_radio_JIKG1_CWM.zip

https://www95.zippyshare.com/v/utogQJub/file.html

I9100T_radio_JIKI2_CWM.zip

https://www95.zippyshare.com/v/I1ajXj48/file.html

I9100T_radio_JJKF3_CWM.zip

https://www95.zippyshare.com/v/SOyKgQ6p/file.html

I9100T_radio_JJKI2_CWM.zip

https://www95.zippyshare.com/v/rtvlHLdv/file.html

I9100T_radio_JJLP8_CWM.zip

https://www95.zippyshare.com/v/N3MI1LWN/file.html

Modem_XXLS9-CWM.zip

https://www95.zippyshare.com/v/jEAPCJ7T/file.html