I was given an 30+ yr old Bike Bug last summer. It was stored with gas and need every rubber part replaced, a new fuel pump, fuel filter, fuel lines, primer bulb, new drive wheel, etc. It would start, but then it would quit making spark. Bad magneto, needed new CDI electronics. It took me all summer and over $175 worth of parts to fix. In the end, its a 23CC engine that was underpowered, but was still fun to ride. I weigh 150 and had it on an older women's Schwinn. On the other forum search "bikebug" for more information. If you want a "project" motor...well this might be it.
This guy is an expert and helped me fix my engine. Contact Mike Schnieder for help Bike Bug
Ps I sold both the bike and engine for $180 this fall. Had fun and didn't lose a lot...good definition for a hobby.