I rebuilt one about a year and a half ago. I already had a gasket kit, but that might be one of the harder things to locate. Rings, bearings, cam bearings were easily located by the machine shop that I sent the block and crank to. The rings or bearings were Federal Mogul, and I think Clevite still has them, too. When you know what size over you need, check out mpcentral on Ebay. He has an Ebay store and he sells the bearings and cam bearing for the P40. Also check out schluderm on Ebay. He has valve stem seals sometimes, and several of the gaskets.

 

Most of the engine hard parts are the same for the P40 motor which is still used in forklifts, so those parts can be found. The P40 gasket kit will mostly work, I think maybe the crank seal or front cover gasket doesn't work from that kit. Nothing major.

 

I doubt you'll need to do anything with the valve guides - mine weren't worn when I did my engine. Valve seals definitely. My engine had 97,000 miles on it and there was surprisingly little wear on the hard parts. The pistons were still within factory tolerances, so I didn't even have the block bored, just honed, and used standard size rings.