look for an old Mongoose
My current (and only) mb is built on a Trek mountain bike frame with no suspension. I chose this bike because it was cheap used, in good condition, it has really good cantilever brakes, and the main triangle easily fits a 49cc motor. Basically it was a good quality bike that spent 10 years in someone's basement with almost no use. Unfortunately, it predates current component sizes, and has a 1" steer tube. However, I recently saw a similar vintage Mongoose with a 1" steer tube for sale for $10 at a garage sale. It was rusty, and I don't think the shock has adjustable tension, but for road riding it's just fine. All I have to do is get about 1 inch of threads cut at a machine shop because the Mongoose is larger frame than the Trek, and I'm in business. If you want to go cheap and you've got access to garage sales or craigslist, I'd look for a 2000-vintage low-end mountain bike. You can use the shock fork, and also use the derailleur for a motor chain tensioner...