As far as the physical changes go between the 250 2t and 450 thumper try using search you will find boatloads of information. I heard somewhere that the classes might be changing but don't take my word for it , i heard that the 250 2t would be able to race 250 fourstokes or something like that. Anyways with efi becoming a big hit on many offroad vehicles within the next couple of years most of the large companies should be switching over to efi bikes which sure does beat having to tune you 2 smoke for every little atmospheric change. Many companies like FMF sell aftermarket pipes and silencers that meet most all race restrictions and what not.