I'm in an almost similar situation. I had a belt-drive 48" Bobcat that I loved; the motor went, but I figured the rest of the machine had plenty of life left. So I dropped $850 on a new motor. Ten hours later, the gearbox failed. :-( A couple of the local walk-behind shops convinced me it wasn't worth fixing, even with the new motor.
So I've also got a parts machine, with a low-time 17 HP Industrial/Commercial Briggs and Stratton. That was an upgrade on the original motor on the 48", if you think it might be of use on your Scag, let me know.