My original thought was to get an uncut frame and put it back to stock,
For less than what it would cost you to do that, you could buy a complete, restored or original '72 Commando in really nice condition, ready to ride. What you have there doesn't even qualify as a parts donor. The engine/transmission is almost the only original part you have to work with. And unless it looks a lot better on the inside than on the outside, the cost of restoring just the engine will astound you. Many special service tools and Whitworth wrenches required. Prices for the frame and other missing parts are worth a king's ransom.
but my dad has already fallen in love with the bike as-is.
My advice - let him have it. Right now, and as is. Let him throw
his money in the toilet. Spend yours on something you'll get pleasure from. These choppers were total crap, and it won't take long for him to find that out. I think your Grandpa found out pretty quick.
Stu