The next step in our ATV
repair saga involves installing the repaired rear prop shaft pieces
back onto the drive gears.
Once we've got them on I'll
need some sort of brace to absorb the impact of banging in the final
pin.
An old scissor
jack might do the trick.
If you can get hold of a small C-clamp, that can help you get assemble the spring pin in a controlled manner.
Personally I don't like spring pins all that much. Until you get past the chamfer, they have a tendency to jump out if you're hammering them in. So if you need to use a hammer, put the vehicle on a tarp or something so that you can more easily find the pin if it decides to take off.
Since the splines on the shaft transfer the force, and the pin is just there to keep everything in place, you could probably also replace the pin by a bolt (preferably a well-fitting shoulder bolt) and a nylock nut, assuming there is enough space. It would be a lot easier to disassemble if required.
You might want to clean the splines with a light abrasive and put some lithium soap (aka "white grease") on them. That will prevent them from corrosion in the future.