I have a pair of MPI's in my hustler that run very well. I know valve springs are an issue on the 500's, but I have not heard of them having an issue on the MPI's. any issues to keep a watch over as the hours add up?

Exhaust seats. I've heard they should be replaced. Have a friend where one actually fell out.

A freind of mine had about 700hrs on his when he blew a head gasket. He did a valve job and new gaskets and that was it.

I would say maybe a valve job at 400-500hrs if leak down dictates it.