I'd be real surprised if a machine shop that is experience with agrarian equipment could not get the parts you need.
My uncle wanted to rebuild the motor in his old Case DC; it wasn't running right. When I pulled the head bolts, it was obvious why. Some of the bolts weren't much more than finger tight. And one valve spring was broken.
We pulled the head and brought it to the machine shop in the city (Worthington, pop. 10,000), and borrowed a hone. The head gasket has seen better days, but we cleaned it up best we could and gave it a couple coats of metallic paint. Some of the rod bearings could use replacing, but for the amount he runs it, it wasn't worth the money.
We pulled the pistons. I removed the old rings, look the pistons over, and just scuffed 'em up a bit to remove the worst of the carbon. While honing, after the second cylinder, I was putting a nearly perfect set of cross-hatches on the cylinders; I ended up honing 6 cylinders in that 4-banger.
Since we were in there, we cleaned the oil pan, sprayed everything down but good. , cleaned the pan again, then soaked everything liberally with oil as we reassembled.
Torque specs? Hah! "Snug" is the term used in the owner's manual. But after tightening a couple rod bolts, I got to 'know' the right torque and fell into a groove: tighten, insert cotter key, repeat.
Got it all put back together and the bloody thing wouldn't start. Being a dieselhead, I've forgotten nearly everything I ever knew about gassers. Fortunately, his daughter and SIL were just, *just*, returning from their honeymoon. "Yo! What's wrong with this?" Turns out I put the distrib in 180 out. OK. Fix it. Still nothing. Check some more. Dipstick that I am, I installed the wires on the new cap one-off.
Finally it starts, belching blue smoke while the oil burned off. And once we figured out that the *throttle*, not the choke, makes it run faster, it was running like I've not heard a DC ever run. (Remember, the DC is 'governed' to 1200 RPM; it was odd to hear it turning 3000 or so. ) The rebuilt engine was (and still is a couple years later) running better than even the one in the DC his Dad originally bought.
So check your machine shop. They should be able to get the main parts you need, especially since you don't need period and true OEM parts. *Then* look at the antique supply outfits.
Attention: TDR Forum Junkies 
