[Upgrading, Modifying and Unlocking] moctir:
UPDATE 2021

The original post was for the first King Kong Mini, for instructions for the King Kong Mini 2 go to page 9 in this thread:

https://xdaforums.com/t/installing-custom-rom-on-cubot-kingkong-mini.4050815/post-84445303

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The Cubot King Kong Mini was released in Q4 2019. It is one of the smallest Android phones on the market with a 4" screen. Like many cheap Mediatek devices, one cannot be sure of how "safe" or "secure" these devices are, so the security/privacy conscious may want to consider installing a custom ROM. Here are the instructions for installing Android AOSP on the Cubot King Kong Mini. Note: I am not responsible for any bricked devices.

Download the Stock Rom from here and SP Flash Tool just in case you need to reinstall Stock. Download a system image, note you need to use an A/B arm32 image with 64 bindings. For example AOSP 9 is here.

This process probably can be done with SP Flash Tool but I wasn't able to manage it due to the disable-verification flag required.

Unzip the stock ROM and reboot the device to bootloader, connect via USB and unlock via:

sudo fastboot flashing unlock

Note this will erase all data. Then on your computer navigate to the folder with the unzipped ROM (for me was NA_mt6761p_d936p_9101cmp_chuanqi_cc_256gbitp24d3_p_lte_3m-fdd-cs_mul_20191121-210020_songlixin_PC) and type

fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img

If you do not do this it seems to work without it but then it will bootloop when you try to switch it on. From here on it's standard.

fastboot flash boot boot.img
fastboot flash recovery recovery.img

Unzip the .xz image file you wish to use and type:

fastboot flash system SYSTEM.img

where SYSTEM is the name of whatever image downloaded.

Root

For root install magisk manager and you have to patch the recovery to get root (not just boot as flashing boot alone doesn't give you root). So in magisk get it to patch you boot and recovery images and then put those on your computer, go to bootloader and use:

fastboot flash boot magisk_patched_boot.img
fastboot flash recovery magisk_patched_recovery.img

You can read more about magisk from recovery here.

To get root then you need to reboot to recovery, hold power button + volume up and select Recovery. This will appear to boot as normal but you will have root when you do it this way and not when you boot normally. Unfortunately I do not have reception when I do this but do have root, other users have this problem but I have not found a solution.

Note that I thought I bricked my phone a few times before getting this as nothing would happen after a failed boot and I couldn't switch it on, however what happens is you can't do anything until the battery has almost completely discharged, then when you hit the power button you'll get the no battery sign and when you plug in you can get to Recovery again. This is quite annoying if you've charged it quite high as you'll have to wait a long time to discharge and be tapping on the power button a lot!!

What doesn't work

Battery usage doesn't work, this can probably be fixed following this guide. If I get time I will attempt this.

Other things to note, there is no TWRP yet so I don't have decent recovery options, this also makes it harder to use certain ROMS as you can't flash things like gapps if you install a ROM without it.

The battery life is definitely worse using AOSP GSI than stock, maybe that vendor overlay would help with that though.

I hope that helps others.