you might want to talk to minidragbike. 20hp for $800 might be hard to do with new stuff. i was looking for 20-23hp on alcohol for my GX200 build before i aborted it and it was going to run me over $1,500.
you'll probably want to start with a real GX200. just a new case (no crank) will run you around $150.
there are 20+hp clones, but a Honda case/crank is much better quality. if you're going to spend a bunch on go-fast parts there's no reason to skimp on the case/crank.