Thats what I was just talking about- I was going to try it and someone above said the bottom join was different- a few years back, the bottom ends WERE different but I measured both my motors and externally they look and measure the same- and BGF advertises his needle bearings, piston pins and connecting rods for both 50 and "80"cc- (66)
I think they may be standard now but I'm not taking both apart to measure the bottom fit- my older 66 motor just happens to have 6mm studs and thats what BGF has for his "upend"
Motorized BIKE GAS ENGINE - 80cc upend 6mm | eBay
I'm not gonna take the gamble but the extra power would be nice if I were going to replace the 50 jug again- for only $15 more
Maybe you could tell from the bottom jug gaskets...