Since its the last oval rear glass one I would keep it as stock as I could.I had a 58 36hp in the late 60s as a second car.Fun to drive.Lots later motors will bolt right in if you want to just be able to change back to stock and run more power now.A 40hp like my 61 felt lots more than four more horse.They got a gas gauge in 62.Had the floor board motorcycle like lever on both mine.

I think you should always keep and upgrade the original when its avaliable. Although modernizing a antique is always cool.