The 65-70-"80" cc engine kits (there are no real 80cc engines, they are all 67.5cc's) are all ABOUT the same...made in China, dubious quality control but lots of fun if you are into tinkering with stuff. The rack mounts I can't help much with other than to say look for one's with Japanese engines like Honda for starters. You do have to watch for things like "Honda designed"- which is a crock, or weird/no name brands on the engines.
Right now I am running a rack mount, which is 28 years old with NO PROBLEMS, other than it's pretty slow- 22 mph tops speed. It's called a TAS Spitz, they were also sold under the names Free Spirit (Sears) and Bike Bug.
The engines do all come "put together", except for mounting them on the bike, but the directions for the frame mount kits are good only for the bottom of the bird cage....that where we come in...the best directions are right here on this site.
Start here- http://motorbicycling.com/f30/bicycl...-care-354.html