Thanks Cabinfever. That looks like what I'll be buying. I talked to the owner on the phone, and what they charge seems a bit ridiculous to me. If I were to buy everything but the engine, my total cost would be ~$620 and if I bought the motor too, it'd be $700. So I guess I might as well buy an extra motor for $80, right? These things normally retail for $400.
Edit: Whoops, that other one is a friction drive. Does anyone know if I can find a complete chain drive kit for a GXH50? The one mentioned above isn't meant for a GXH50 and I don't want to pay for extra parts that I won't use.
Also, are friction drives worth it at all? It seems like they might slip a lot, and damage the bike tires. Has anyone used a friction drive and found it to be worth while?