finding a proper engine

hey guys, I'm starting on a new project. I'm making a motorized giant stiletto. with the jackshaft it comes with (which is really just a bottom bracket with gears on it), It seems like it is going to be a nice and simple project. The problem is finding the right engine for it. I'm willing to spend up to about $200 on it, probably used. I will be using it for regular transportation in the hills of WV, so I'm thinking towards reliability and an ability to climb hills reasonably. I found what I really want, a Yamaha ysr50 engine (or rx 50 engine..same thing). It is a 49.3cc 2 stroke with a 5 speed transmission. I think adding an engine with a transmission will be best, and It'll probably have a jockey shift. Fun right!! Problem is their hard to come by for my budget. any ideas for any similar engines I could use instead, or something better?

just remember, it can't be more that 50ish cc's because I'm not plating it and thats the legal limit around here.

well I already have the typical 66cc 2 stroke in another bike, but I'm not convinced that putting a shift kit on it is the answer. I haven't seen one up close though, what is the tooth ratio on the easiest gear? If I were to do that, I would build my own. The giant stiletto already had a shaft going through it you can use to hook the engine up to the cassette. it would be a 10t engine cog running to whatever gear you mounted on the left side of the shaft, going to a 20t on the right, straight down to the cassette, the largest gear on that is about a 34t. does the regular shift kit give you better ratios or just connect you to the drivechain?
My idea was to keep the bike in 1st gear and use the engine transmission. Its high enough that you can walk beside it in 1st gear. btw this is a 24" rear wheel so it will have a little more torque that a 26"

TLDR: unless there's more to a shift kits I'm missing, I don't think I'd want one.

Thanks a lot for the imput guys! I'd love to hear more (and correct me if I'm wrong)

well It may be wishful thinking that I can even build one to climb the hill on my route. its a real monster! one of those kind where you think your car is going to tip over because its so steep.

so I found the first engine I'm going to try. A kid up the street just sold me a 1980 honda ct70 (4 stroke) with a 3 speed semi automatic transmission. it's missing the top head, carb, and one side plate. but beyond that pretty much just needs assembled. Picked it up for $20 on the promise that he didn't have to see it again.