Good question, I acquired a blown up GX630 and sort of looked around. http://www.parsonsracingengines.com/ has some parts. I like the oil pumped crank and cooler it has. Mine had aluminum rods yet I think Parson's are steel. I think the GX390 uses the same pistons.
You can make your own parts. A lot of folks do it. You got to learn a bit about sand-casting and lost-wax casting. You are going to need a metal smelter as well - can get expensive depending on the types of metal you intend to melt.