Thanks. Pleasanton,CAIf you get the gears shifting, you will want to set the gearcase to neutral and the control to neutral, for reassembly. In my mind that reverse lock is optional. Just don't quickly rev up the motor in reverse, and she will stay down, well enough.
If you cannot get the gears to shift, you probably will need a used gearcase. Price out the gears and clutch dog, and you will see high prices, if you can get 'em.
Where are you located?