If he wants electric start and you have most of the parts then it shouldn't be too difficult to make it all work.
When you say "we are thinking of mounting a 38 inch Ariens deck I have here on the back "
I assume you mean a tow mower and not directly mounting the deck onto the back of the quad bike because obviously you can't mount the deck directly onto the back of the bike as this wouldn't work with the suspension.
I'm sure you've already thought about the ride on mower deck having linkage so it moves independently from the quad bike.
You also don't necessarily need swivel wheels as you can make steering front wheels on the front of the tow mower that works off the trailers towing arm.
