I've welded up a couple spots on my 500 frame and re-enforced some shady looking spots from factory. I haven't had any issues at all. I've had some hard crashes and run hard through whooped out trails. As long as the weld looks solid I wouldn't worry about it. Don't think that just cuz a weld is pretty that it is solid. I've had the prettiest welds break and some of the ugliest hold forever. It's all about how well the weld penetrated the steel.
Glad you kept it. They are very fun and dependable. I completely rebuilt mine (all factory specs) and it starts every time and runs excellent. It's also fun to pull away from the punks on the new 4 stroke thumpers.