What do you think the best way to go on a lt250r would be to increase top end speed ????? any1 know where to get a 350 big bore?

 

check out LRD, CT and trinity... in that order

 

Don't forget duncan racing. Personally I would stay away from big bores. Do to the fact of relieableity (sp) issues and more expensive parts. Plus don't forget cost of race fuel.
I would port and polish the cylinder, add a pipe system, bigger carb., rad valve and misc things like that. Call one of the engine builder and talk to them and tell them what you plan on doing and what type of riding/racing your doing and they help you to make a better decision on what to do with the engine.