Parts are available in the form of raw stock for the pushrods and machined pistons. I'm not sure if they are available with threaded bores or not.
At some point I'm going to be buying from http://www.crconline.com/
I want to experiment with making some welded cylinders in custom sizes. The design of these garden tractor cylinders is just about as good as it gets for minimizing the closed length and overall size.
I haven't figured out how to mount them up in the only lathe I have access to so I'm doing it crude, cutting the base off with a angle grinder. Not pretty but it works.
It's hard to hold them in the vise, sometimes high spots on the weld don't allow solid clamping. A strip of lead flashing wrapped around the cylinder makes the problem go away.
I haven't welded one back together yet. I had bought a TIG torch, planning to do a nice root pass with TIG. Unfortunately when I fell and wiped out my shoulder 13 months ago it put us on a really restricted budget here so I haven't had the money to get the TIG consumables and buy or lease an Argon cylinder. I'm using flux cored wire in my MIG, can't justify the cost of gas for that either. Flux cored is a bit fussy to throttle down but I think I can make it work in this application. Given normal conditions I would MIG it with .023 solid wire and sew the root with a nice small bead, then follow up with a couple fill passes.
Either way it will be a slow process by necessity of keeping the cylinder cool enough to not burn the new innards out of them.