Rich Rohrich said:The TTR250 cylinder head is horrible and even with lots of work it will never flow enough air to adequately feed an engine that size. A $500 FCR carb won't change that. Cam selection is also fairly poor so you'd end up with a heavy bike fitted with an underpowered engine that stops making power way too early.
As cujet pointed out, building a WR290F or a WR305F is relatively cheap these days with the availability of new cast cylinders from Athena (290) and stroker cranks from Hot Cranks. You wouldn't have the simplicity of air cooling, but in all other respects you would have a vastly superior bike.