There are still parts available for that 3-speed transmission, and good diagrams on boats.net. So repair is an option. Damn things are bulletproof, they only problem I've had is when the nylon key holder assembly swells so it gets stuck in one gear, but its an fix with sandpaper.
The same gearbox, just with a longer axle, is on my hra215 (hr215?). If you find another comparable transmission (ie, identical gearbox, different axle length) you could simply open both units up and swap the axle. May take awhile to find, but I've seen clean units on ebay for <$100, on occasion.