amateur liquid bi-prop rocket engines for the High Power Rocketry hobby are gaining momentum. There's lot of opportunity there for performance profiling and even more if you have access to a machine tools like CNC lathes. There's also interest in active stabilization of amateur rockets using engine gimbaling which would put so much more performance in reach.

These guys are legit and actually flying airframes instead of just ignition on a test stand. https://www.halfcatrocketry.com/

The hobby is geography constrained though, you need access to large open spaces. Even small engines are spectacularly loud and igniting one in your garage would scare the crap out of your neighbors.

Edit: if you're in/near LA this club is pretty much ground zero. Tom Mueller of SpaceX's Merlin engine series fame was discovered here iirc. https://rrs.org