If you're after good numbers with a stock engine you want to find yourself a decent Mercury 11x18p cleaver or 10 3/8 x 18p chopper. The choppers were originally for 10 spline 25hp but most have been resplined to take 13.
Both of them should give you about 56mph when set up correctly.
Yamaha make some good props too, 19 and 21G cleavers, which are also very good, although slightly tall for a standard 50.
If you're after even top speed (you will be
) then you could go for a 20p cleaver, still 11" diameter. The acceleration will suffer but you'll be going close to if not 60mph then.
Genuine merc props will always work well out of the box, although you can fine tune them to your application. You may have heard of Ron Hill, he makes all sorts of props but they're a bit of a gamble. Some are stinkers and some are beauties. Upto you, but I'd always go genuine merc props unless you buy a used prop that you know is balanced and works.
Unless your crew weight is very light 10.25" diameter is not ideal for a cleaver. It works on the chopper because it has over hub exhaust, so more blade area.
If you just want a bit more bite and slightly better top speed then do as Luke says. Money spent tuning propellers is always better than money spent tuning the engine.