I wanted to do this for Ludum Dare and some people gave me grief about it, even though it's legal by their rules.
My AGPLv3 code is here: https://gitlab.com/activated-onion/ld42-warmup
Here's a screenshot: https://social.freegamedev.net/photos/activated_onion/image/d2286632f9b14c30b5c3...
There's no gameplay, it's a graphics tech demo with physics. By LD rules I would make new art, sound, levels, and any game-specific code. The physics, input, file loading etc. low-level stuff would be reused.
Is that okay for OpenJam? I've gotten used to using my own tech and I don't feel like using anything else.