No problem, but I would encourage you to reconsider your train of thought.

The LPK25 is limited, but it's definitely more convenient for getting down ideas than using a computer keyboard, since once you have it set as the MIDI input device in an application, you can hit keys and play notes without having to worry about what application is in focus and what its keyboard shortcuts are. Computer keyboards also only recognize up to a certain number of keys being held down simultaneously (since they're designed with the assumption that you won't be pressing five keys in the same row at the same time) and don't provide velocity or intensity information, but you always get note intensity and velocity MIDI information out of a MIDI keyboard. As for the limited number of keys, you can shift the keyboard down or up entire octaves with the buttons on the left and then record the left and right hand parts separately in two passes. You can also look at it and see what notes you're pressing instead of having to translate between a computer keyboard and a musical keyboard.

So I'd say it's quite useful compared to having just a computer keyboard, and the small size means you're that much more likely to have it with your computer, even if your computer is a laptop and you travel a lot. Your argument that it only does what you could still do with a computer keyboard is like saying the virtual keyboard on an iPad is just as good as using a bluetooth keyboard, since they offer the same keys and basic functionality... so a bluetooth keyboard for the iPad is useless. But you'll probably still want to use that bluetooth keyboard with the iPad if you're doing any amount of writing, because it just makes it easier to type (and it frees up the screen space for what you're writing).

If you have the desk space though, Behringer makes some cheap gear that usually works fine, so go with a larger keyboard and check out the Behringer UMX610, which should work perfectly with Linux:

http://www.bhphotovideo.com/c/product/672870-REG/Behringer_U...

You could even use such a keyboard for actual performances if you absolutely had to, but it probably doesn't play as nicely as your Casio, and you should probably figure out your latency problems in Linux. (To be honest, if I were doing anything music related on an everyday basis, I'd much rather use Cubase on a Mac. Linux software is a disheveled mess of parts that can fit together quite nicely, but can also cause a lot of hairpulling too.)

Finally, you probably don't want to hear this, but if you have time and energy for ear training practice, relative pitch recognition would make your life a lot easier and solve most of your problems. Then if you can work out what the first few notes of your song are and you figure out the key, you can figure out how to write down the rest pretty easily from there. Then you could use some of the score editing software, trackers, or anything else that people mentioned in the other replies.

As it is now, it sounds like most of your musical fluency is trapped away in the muscle memory of your hands, and that's the real source of your current problem, since it's the same as being able to speak and listen, maybe even read a bit, but not write anything.

Anyway, just something to consider. Good luck, and I hope you find something that works for you.