Double/triple check your carb cap to be sure the threads aren't just stripped inside the cap. They are easy to cross-thread, and you may have just damaged the cap threads. In which case, you can replace just the bad carbuetor cap and go on with your project.
There is no known (good) way to fix stripped threads on the top of the carb itself. That said, stock carbs are a dime a dozen, and if yours has been damaged beyond repair, I'm sure somebody on here would hook you up with a good used carb for next to nothing.
Keihin PWK 28mm carbs are a nice upgrade over stock for a Blaster engine. They bolt right up, use the regular TORS eliminator throttle cable, and can be had used off a KX 85 for around $ 80.00 if you look hard enough.