Can you put a 300ex crank in a 250x? I'm looking to stroke my 250x to go along with a big bore kit. I heard that a 300ex engine is the same as a 250x, except for the electric start and the crank. If it can be put into a 250x, are there any other modifications that I would need to make?

 

From what I have gathered through other's experiences...you can't directly swap cranks from these two engines.

Never attemped this myself, so I am not sure what the fitment issues are, but I have seen several posts regarding this over the years and I haven't seen one success story yet.
I would only assume that maybe the left side is different in many ways to accomodate the starter clutch on the 300EX.

I would get the 250X crankshaft stroked rather than fooling around with a used 300EX crankshaft of unknown condition.