The old one is probably running 2:1 with a 13" prop-gearbox setup,whereas the new one may be setup at 2.25:1 on a 13". Course somewhere back there OMC went to the 10" prop-gearbox and ran it at 1.64:1. If you both are running on similar boats, No way will 12 ½% in gearbox change span that disparity. 22 pitch on a 50 hp engine, if they even make one for the 10 inch (prop dia) gear boxes would be running on an 8' tunnel hydroplane or something light and fast, single person minimum weight installation.......buttttt the prop shaft on the 1.64 is turning much faster than the 2:1 so it would be more of a candidate for the 10P for a given load to push.
Agree with racer. Get some prop numbers, on the rear of the hub usually....pull the prop nut and look.
