You could use an FH721V-GS24-S replacement but it would have the bendix type electric starter. If the starter off you current engine is fine, you could just swap the starter. Or with the bendix type you would need to install a starter solenoid on the mower. The closest direct replacement with the same shift-type starter would be FH680V-GS28-S. This would be a labeled 23 horse power opposed to the FH721V 25 hp. You would never notice the 2 hp difference as the way the hp was rated on the FH engines, it is possible that both engines only made 23 hp, for example.