I am on the heavy side myself. I went to a friction drive because I was almost killed by one of those Chinese chain tensioners getting pulled into the rear wheel. I have a Honda GX35 4 stroke. I do have to pedal to get it started, but once up to speed it will pull me along nicely at around 20 mph on flat ground. I'm in AZ, and the MB speed limit here is 20 mph. It needs a little help on steep hills. A smaller roller would help there, and I may change to one. I don't really need to go over 10 mph, which is about the speed of a pedal bike around here.
However, I may be doing another Chinese engine build in the not to distant future, as a couple of products have come out since my crash that solve the two main problems with the Chinese kits. An engine mounted chain tensioner, and a clamshell rear sprocket mount. Yes they are expensive by the standards of this forum, they would add almost $60 to the build. But I expect the total to be close to $800 by the time it's all done, including the cost of a high quality used bike from Craigslist. Older name brand steel framed mountain bikes with small diameter frame tubes work well, and already have good front brakes. I convert them to very low geared single speeds.