What is the bore? Don't know. I have never had the engine apart.
Ever find out compression? 115 left, 110 right.
Is the crank welded? Don't know.
Is it trued? Don't know.
Is it coming apart? Please take a look at the youtube links I provided.
The clutch, is it good? It shifts, disengages, and engages with the quad on a jack. I have never ridden the quad.
Have you done a leak down test? No. I don't have the tools to seal up and pressurize the engine to check for air leaks.