RE: Decent motor and clutch for $100?
I think 40mph is not going to be possible with out some serious mods.
Unless you get a tuned pipe on that motor, its not going to have very much power.
Its probably a 1hp engine maybe 1.5hp.
I would say your best option, is to find a used zenoah, new a 260pum can be found for $200 so you should be able to find a used zenoah g23, 231, 261 or the like for under 200 with mounts and maybe even a pipe.