A lot of Cub folks are "into" the dash 8. Don Swords even got approval to convert perfectly good dash 12s into dash 8s. Find one of those, and trade "up".
I am 76, and still quite capable of hand starts. I did two today. But I foresee the day when climbing in to a running Cub could present risks. And my partner, who is barely into his 60s, is having back trouble. So we sort of got ahead of the curve, and did both Cubs last winter.
The B&C fits, but no room to spare. The Bendix S-4 mags fit, but those little nuts that hold the wires in the back of the mags need to be shortened. One can use modern swaged spark plug wire and get more room.
The B&C factory is easy to work with. Still locally owned, as opposed to Skytec. And the product seems bulletproof.
We are using the Odyssey PC-545, a smaller 12 AH battery. It is clear that even that is overkill - if we could find a 7 AH with robust terminals, that would be the ticket.
Good luck - we know where your buddy is coming from.