The frame cracking is very common. If you search the site you'll find lots of threads about guys adding gusset plates all over on the frame to reinforce it. There have been a few members here that offered a kit of the pieces you'd need to weld in. To do this requires a complete teardown of the machine which may not be what you're after. If you just want to fix the one crack, a weld alone probably won't help much unless you are a very casual rider (low stress on the frame). If you can add at least a couple of the gussets in the vicinity of the crack as part of your repair, it should be a lot more durable.
Flipping the machine all depends on how much you can get parts for and more importantly what you are gonna fix and what you are gonna let go. If you fix everything on the machine, do it right, and use all original (used from e-bay) parts, my experience is no way you ever recoup your money invested. $800 is a great starting point, but the $$$ add up quickly when fixing everything up on these machines. That's been my experience, but I get pretty picky about making everything right. If you let some stuff slide and the buyer isn't picky, then maybe you can turn a profit.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70