Whether the shaft is splined, has a roll pin, or threaded should make no difference. Get a compatible pipe / shaft and extend it. Regarding the difficulty of finding any given gear, it's a toss up. Some are great and others not so great. Take a peek under there and look at the gear shift rod. You might notice some slop that can be easily addressed. If not it would be internal which I guess is not something you want to get into. Can you not select one gear for downhill and another for uphill to mow in a safe manner? I wouldn't be real trusting of the brake in a downhill, runaway mower situation. Make sure the brake is fresh and adjusted and especially make sure the drive chain is in good condition and that it's tensioner is properly adjusted.