The airspeed is going to vary a lot with things like fuel, weather, and where you fly. The cruise speed doesn't really have to change, the plane will float along in a slow, stable cruise with either prop.

At full power, a stock Alpha 60 will usually go around 45-55, I'd guess. Depending on prop, you could probably push that to the upper 60s, but I haven't tried playing with props for max speed, it is a trainer after all, and trainers often fly better and are more fun with a larger diameter, lower pitch prop for better climb and acrobatic performance while keeping the max airspeed down. Espeically because most trainers don't have control linkages that are really set up well for high speed flight. (I've not looked at an Alpha 60 from the perspective of making it go fast. Again, no real point).

As for a prop to start playing with, a 12x6 APC seems to be a pretty good place to start, but you'll want to experiment.