The only thing wrong with this starter motor reverse thing is there is no speed control. Cant help thinking that it would be a complete ballache trying to reverse that into a tight parking space that needs a shunt or two. There wont be any feel to it , on/off thats it and if you ended up with a 3.1 diff with that size ring gear on the prop your reverse is going to be very rapid
Just thoughts
Bob
Just put that lot into a speed calc. and thats 32.8 mph reverse gear on a button