In my opinion the ideal motor for a Jupiter is a Johnson/Evinrude 3 cylinder 70/75. Excellent power to weight ratio. I think Yamaha made a 3 cylinder 90 that would also be pretty sweet.
I had a big V4 85 on mine and I think the added weight isn't worth it for the little bit of added HP.
Hey Andy what kind of 50hp motor and what year is it? Mine has a 1972 50hp Johnson which needs to go.
Hey Andy if you were to repower your jupiter what would you get?
ezmobee says his jupiter did well with a 3 cyl 85 hp.