Hey Bluecar,
Looks like a nice score to me. When I was a kid and BMX was just starting, we would BMX anything we could get our hands on. We quickly discovered that Schwinns could take the abuse and everything else broke. Schwinns had the nice filleted brazed joints. I personally have yet to brake a Schwinn and your frame appears to have the same tube joints. That said, there are a number of tricks to smooth out the two stroke engine and drive train. If you do your homework ahead of time and don't rush the build you will have a fairly smooth running machine.