I am assuming the 350 is a 2 stroke. If so is the bottom end interchangeable with the 250?

My brother and his friend lost the cases when they attempted to rebuild our four wheeler...

 

The 350 is a water cooled motor and the 250 is an air cooled motor. Not anything there that will interchange. The 250 and 300 are very close and the 350 and 400 are very close. Many parts from one to the other will swap on those. But not from air to water. Hope this helps, Bubba

 

That does help me. Thanks for the info.