Sounds like a dandy idea to me. How many Sixties have we seen outfitted with pony motors? I have seen one of the very first Holt tractors built in Peoria outfitted with a pony motor. My humble opinoin is if it is done well with good workmanship I don't see much of a problem. There will always be purists that will not be satisfied with anything but 100% origional. There is a strong inventive spirit with the people who worked with and on these old Cats. The people who go to the scrap pile and grab a hunk of iron to weld on a crack, or ones who cut giant holes in sheet metal to get to a difficult bolt, Now those are the folks that ruin good tractors.
I've added a starter motor to a couple of D4 7U's so the only time I use the pony anymore is when I want to annoy the neighbors Here in the hills of eastern Oregon, I've never had a problem starting 'em in winter. .just a sniff of ether while cranking and they've rumbled right to life.