Changing out a finish mower gearbox isn't a big deal, "IF" you have plenty of metal fabrication tools and experience to use them.
Most gearboxes simply sit on a platform, so the height of the platform, the gearbox bolt pattern will matter. You will have output shaft dimensions issues, which leads me to want to buy a used gearbox with the pulley included. However, getting the pulley off can be an issue. (I have a couple Rhino finish mowers that I have NEVER been able to remove a pulley. The last time, I just installed a new Taper-Loc pulley.
I would look for a way to figure out what mowers those gearboxes were used on, and see if I could locate a used mower that the deck had rusted out on. Most finish mowers get parked for reasons other than a bad gearbox.
David from jax