alpha gear change for increased top end or new engine?

New to the forum. Have a 1992 20′ Celebrity bow rider with a 5.7/260hp Mercruiser. Boat currently goes 55mph (speedometer) my guess is more like 51 (gps). My goal is to get this boat at or over 60mph. Originally was thinking new Vortec “roller motor” rated at 325 “crank” hp. Been doing some alpha gear change research recently and was wondering if this might pan out and be less expensive. Current alpha (gen II) ratio is at 1.62 and if I went with 1.47 this (on paper) looks to get me to 56mph. Would be a nice jump in top end speed for cheap, (under $500). Found this formula online which seems legit:
rpm/gear ratio x pitch/1056 = top speed. My info: 4600/1.62 x 19/1056 = 51mph.

Any thoughts or input much appreciated!

Dave