Okay, this post is amusing me. I've got to see this franken-bike work. Even better if I get to see the gas motor bolt on.
You can find a 90 tooth sprocket and a 9 tooth sprocket here:
http://www.electricscooterparts.com/spr ... chain.html
You'll get change from $37 for the two of them together. (Have to pay for shipping still though).
I count 11 teeth on an MY1020.
11T x 3000rpm = 33,000 teeth per minute (tpm)
33,000 tpm / 90 teeth per rotation = 366 rpm
366 rpm x 9 teeth = 3,300tpm
3,300 tpm / 32 teeth per rotation (Largest gear on a derailleur) = 103rpm
3,300 tpm / 11 teeth per rotation (Smallest gear on a derailleur) = 300 rpm
Assuming 2m round for an average sized 26" mountain bike wheel, that means, in lowest gear 12.3km/h and highest gear 36km/h
That's your unloaded speed. You could probably use a 80/9 to get about 40km/h, or maybe a 90/11 to do the same thing. 1000W gets me to low 40s, but at 400lbs for the bike and you combined, your mileage (and speed) may vary.
I'm just imagining a 90 tooth chain ring buzzing at 366rpm (if that doesn't sound fast, just think of the 550 teeth passing you every SECOND) right behind your butt while riding though. I'd be like you're being chased by the Texas Chainsaw Massacre-er, except, the faster you go, the closer he gets, and his chainsaw is going faster.
Oh, this is going to be funny.