Today I will add the capability to push the Keyboard Scale past 1 in Keyboard Settings - you can look for Version 1.14 on the website in the next 24 hours.
This will allow you to zoom in on the keyboard, removing the extra keys at the edges.
I think you may still need to do some video cropping outside the SeeMusic app, but that shouldn't be too difficult.
I don't think I would be able to create exact custom keyboard layouts for all different keyboard sizes - that would be very difficult, if not impossible. You may happen to have 61 keys, but other people have 49 keys, etc. So scaling up the keyboard would probably be the best solution for now.
Could you help me understand better what you need? I don't fully understand what you're asking...
Your app is made for 88 keys keyboard. But unfortunately, I have 61 keys keyboard. So is it possible to make 61 keys synthesia?
There is one option I seen which allows to select keyboard size. So how can I import 61 keys keyboard?