Hmm, was the motor ever rebuilt?,, Broken valves are a sign of not having them replaced. Valves get weak as times goes on, even if they were re-ground, or if your hard on your engine you'll snap valves, you have a very bullet prof engine, but it's not uncommon if you snap a valve on a well used thumper.

I'd also like to point out your engine is nothing like a 450r,, the only thing they have in common are they have a piston and use valves, their designed totally different,
I don't think the carb was the problem, unless it was real lean and heated up the exhaust valve, Post a pic and I'll tell you.

Anyways it looks like you need to start rebuilding,, good luck..