SR250 lump has a starter. The XT250 does not. New motor mounts needed.
I bet money the "blown" part is a failed cam/rocker. The cam may have spun on the cam gear too.
You can replace all that without taking it apart. And from what I have seen in my "blown" SR250 motor, it is non interference if the cam seizes. If so, you don't have to touch the bore.
What is "blown" about it?