If you intend (or would at least like) for other people to play/try out/use what you make, consider going with a Commodore 64 instead of a 128; for people with physical hardware, the 64 is far more popular and will have a wider base of users.

Of course the 64 is a bit more limited than the 128, but handling that is part of the fun of retrocomputing. "It's not the lines that make playing Tetris interesting - it's the walls."

Game boy and nes are pretty easy with modern day tools.

It’s not technically retro but I’ve found Pico-8 a great way to build retro like games! It’s pretends to be a console of a bygone era and gives a bunch of restrictions on size of game and 128x128 resolution.

The app comes with a pixel editor, sfx and music editor, and a code editor built in. The games are easy to share and you can play them on a mobile browser. On the downside, it costs 15 dollars for the app.

Are you looking to run on actual hardware?

If not then you might want to also consider fantasy consoles.

Not old, but retro in a way, Playdate is a pretty fun machine to play with and if you want you can end up selling in their catalog (online shop both for browser and console). I have wanted to start doing something for it for the last year (im a game desginer) but i havent coded in like 5 years at this point and work leaves me exhausted to learn new stuff.

If your gut is saying C128, then start there. Have fun 🙂.