Hi, MikeL
My first MB was a home built friction drive with a Ryobi weed wacker engine. I think they are usually 31cc and it's plenty of engine for your size. I got about 22 MPH out of it and I weigh 170 lbs.
The main problem with that engine is I could not figure out a good way to use a clutch with it because the clutch they use is pretty light weight and probably won't last long.
So, I made a frame to hold the engine and mounted the engine to the frame and found a BMX foot peg that had the right bore and threads (mine was 3/8" x 24 TPI) and threaded it directly to the end of the crank shaft. Then, I pivoted the frame up and down with a long handle so the foot peg, acting as a drive spindle, either contacted the tire or not. I used bungee cords to help hold it down on the tire. I think it's called a 'suicide clutch.'
I copied a design I saw posted here by one of the regulars who took some good pictures of his setup. Sadly, I can't recall his screen name at the moment. Maybe Mountain Man or Happy Valley... I'm getting old... I just can't remember right now.
Anyway, it worked great and I rode it that way for a year. I accidentally burned up the engine by letting it run against the tire on a long downhill section after I ran out of fuel. Without fuel, the engine had no lubrication and a ring broke.
I got another one off ebay for $20 and that engine is still going strong. I also got a couple of Ryobi 25cc 4 stroke engines and used them. They were great, too.
Eventually, I moved on to a BMP FD kit with a Robin EH035 35cc 4 stroke engine to get the convenience of the clutch and the ease of the 4 stroke engine. It's been a great way to go. Right now, I'm building a trike and I plan to use that same engine.
If you need any more info, please let me know. I thought it was a great way to start out in MB for no money at all since I had the engine laying around and I built the frame out of junk wood and aluminum I had in the shop.
I think some guys here have made MB with the engine and the clutch and they might have some good ideas for how to make the clutch last.
Good Luck!!!