Whereas HP ratings were promulgated by the ICOMIA Standard 28-83 (developed in 1983, thus the -83 part of the standard #) and in place by '85 on all motors ... I think going to the newer 50 would be a downgrade, as the 60hp should be a propshaft rated puppy.
BTW, there are some misconceptions on hp ratings between crank and prop ratings, as well as overall hp labeling. Typical losses or reduction in torque is 2-3%, maybe 5% at most, when crank rated. If anything, motors, especially Mercs, were BOOMERs before the change, putting out much more hp than labeled.
OBs hp ratings are also at the mid-point of the recommended range, where the highest power within that range cannot exceed the declared (labeled) power by more than 6% (per section 6.2). So with a labeled 200hp rating at the mid-point of the RPMS (ranging 5000-6000 RPMs) that OB cannot product more than 212hp at 6000 RPMs. I was always under the assumption that OBs had to be +/-10% of the hp rating & label from the Mfg'r as built, but it appears that may only apply on the low end.
Regardless, I think your 60 is really a 60 and I would ensure ... whatever motor you use ... that she is propped right! To me, that is producing no more than 200 RPMs off the maximum RPM on the boat with you and the typical load carried and 1/2 fuel. But that's how I rig my boats.
FWIW my 2 biggest boating pet peeves (that you didn't ask for ... lol :wink: ) are that #1) most OBs wear the wrong prop and that #2) most OBs are rigged and mounted too deep.