Bad fuel pump on a Onan CK-S?

[brooksadsit]

I recently purchased a Marquette welder JG-200-L with a Onan CK-S 259N engine on it for parts. The welder is complete and was running a few days prior to my purchase. Apparently what happened was that it was running and then there was a bang and the air intake blew off of the carburetor with a lot of oil. He couldn't start it again. The crankcase was apparently filled with gas, which leads me to believe that the fuel pump was leaking, and sure enough the fuel pump diaphragm looks in poor shape to me, though it seems to work when I had it out of the machine. The engine is free. So my question is whether I am on the right track? Or is there something else that would have caused this? I plan to rebuild the fuel pump but is there anything else that I ought to look at or do? I would like to have this running to have a spare.