Being both Kawasaki engines, the wiring should be plug and play, or if any modification is needed ,it would be simple.
mounting bolt pattern should be the same as well.
Now, if the Hustler has an Electric PTO & the Cub doesn't, you may run into some issues, as the FR691 will probably be equipped with a 20 amp stator to keep up the battery with the PTO electric clutch., and if the FH721 doesn't have a PTO clutch, it will probably be equipped with a 10amp or so, stator.
The main thing is to be sure the crankshaft PTO end is the same LENGTH and Diameter.