Almost any 2-stroke sportbike will get increasingly more difficult to find parts for as time goes on since the EPA has pressured manufacturers into reducing or even eliminating their production or importing into the USA. Another issue is the number of race classes 2-strokes are allowed to compete in that they would actually be competative, not saying you wouldn't be competative, just saying the other bikes in those classes may have power advantages. A 125 is great for learning corner speed, but it has limited acceleration compared to the 4-stroke bikes in the classes you would probably race against. For that reason you would probably find you would catch up to and get held up in the turns by alot of bikes with more HP, then they would accelerate away from you on the straight's (which is very frustrating). A freind of mine tried racing a 125 at our last race event and said that even though it was alot of work it was a fun bike to race.
A 250 2-stroke is a bit different because they have more power. On a 125 you have to maintain corner speed or it will take you a while to get your momentum back, on a 250 you'll be able to recover your momentum quicker. A 125 requires that you help it get going at the start of the race (as in a bit of Fred Flintstone action), on a 250 that need is greatly reduced. I have only seen a handfull of 250's (even less 125's) being raced in our region, but the guys that have learned how to ride them do pretty good.
Be prepared ahead of time that these bikes are very finicky, need to be rebuilt somewhat often to stay competative, and can be a bear to start sometimes. With progressively less and less people racing 2-strokes today their will be far less people at the track able to offer good advice or help with them, definitely something to consider. Also be prepared to find there may be a limited amount of choices of race tires for these bikes (especially the 125) if their tire size is not the same size as a more commonly raced bike.
I'm not trying to talk you out of a 2-stroke, I just want you to be aware that there may be some unexpected challenges your going to encounter just trying to keep one running good and racing one.
