The snap ring is there as a safety, to prevent the yoke of the U-joint from coming off the gearbox input shaft if (when, as) the driveline retracts upon pin shearing.
The snap ring should not self-destruct each time the shear pin fails. If it does, you have other problem(s).
Ideally, you shouldn't even realize the snap ring is present or that it's doing its job.
I've had one pop off. I know, it shouldn't happen. It did. I guess it broke or otherwise gave up from fatigue. That driveline flopping around on the cutter deck at 540-RPMs will certainly get your attention. After cleaning the tractor seat and reinstalling snap ring you can get back to work. I now keep a couple loops of 1/2-inch nylon rope tied loosely around the driveline from the cutter's 3-point lift-frame. Just in case.
Now to the shear pin: you can use a standard 'no grade' (unmarked head) bolt of the proper diameter. Actually, it's a 'Grade-2' bolt, which is indicated by an unmarked head. Do not use a heavier grade bolt: you want it shear when needed---a Gr-5 or Gr-8 probably won't.
Get a bolt long enough so that no threads are inside the hole---the hole should 'see' only the slick shank of the bolt. Use a NyLock nut made up tight to the end of the threads. Cut off the threaded end of the bolt if it sticking out bothers you (safety: prevent it catching something like a shirt sleeve). Thirty seconds with a 4-1/2-in hand grinder zips the bolt right off.
If NyLock nuts aren't your thing or you can't find the spare you have on hand, use a standard nut and "stake" it by hitting it with a cold chisel where the bolt exits the nut on the top side or if cutting it off using center punch on the end of the cut-off bolt. Doesn't take much and nut is locked on.
I've never liked the cotter key-type pins after seeing cotter keys mangled and almost yanked out of the hole when working in heavy brush. And I've never had much luck with the 10-hole one-length-fits-all pins as they seem a bit delicate compared to a solid bolt.
Make certain all your U-joints are greased and your grease fittings are intact and greased.
Please post back your experiences so we may all learn.