So here's what I think I'd need:
-3-speed lawnmower transmission: $20 + $11.54 S&H from Surplus Center
-7 tooth #41 sprocket with 3/8" or 10mm bore (think I can bore out a 3/8" sprocket to 10mm?) for the input side of the transmission: $6.41 from McMaster-Carr
-15 tooth #41 sprocket with 1/2" bore for the output side of the transmission: $12.18 from McMaster-Carr
-41 tooth sprocket for back wheel: $13 from ThatsDax
-6mm rag joint: $8 from ThatsDax
-55" of 415 chain: $12 from ThatsDax
+ $9 S&H
Total: $92.13 (aka outside my current budget)
Pretty much everything else I already have or can make myself (motor mounts, throttle, chain tensioner, gas tank etc.). But I'm wondering whether it might be better to get a two-stroke bike kit from ThatsDax or somewhere similar, and then sell what I don't need, rather than buying each individual part. Because I'd like to have a real metal tank rather than the plastic one I've got, and I'd rather have a real twist throttle than an old bike brake lever. I might also need wider cranks (haven't measured that).
Does anyone have a cheaper source of parts that I've missed? Maybe I'll put a "wanted ad" in the "Swap and Shop" section. If anyone is in the Denver area I wouldn't mind trading bicycle parts or Briggs & Stratton parts towards what I need.