It is very possible. I've been fortunate enough not to have any major issues. Last race I shorted out my electric start, but that's it. You gotta love Honda. I have past broke down 450's from every other brand except a Suzuki. The only Suzuki that races with us is the same guy who works on my quad.
No, I try to stay away from motor work. Stock does me good and it keeps it's reliability. I have a 2 bro. exhaust, uni filter, and dyno tuned. I hit about 78 according to GPS.
I seen a 2bros slip on at the honda shop it sounded good if they made a full system for my 400 I would by it. but Im gonna probly get a sparks. I had LRD and a Curtis SParks on my 450 they were both awsome but I liked the Sparks more. Bu that 2bros muffler sounded great!