I honestly don't think it gets any easier than what jotd is making publicly available on GitHub as you get an actual source code you can compile, tweak and test on your own.
I'd start with something like DonkeyKong:
https://github.com/jotd666/donkey_kong
But, any other projects he's been working on and put on GitHub would work.
This way, you can at any point, instead of just using a disassembler, check the source code, which is a huge bonus in my book...