Based on my tach readings on my Tower 75, my guess is that you get about 10000 rpm on that 13-8 if it is an APC, which is to low for the engine to produce its best hp. If you want to go fast you also need rpm`s and not just the biggest prop you can find. A 12-8 will be faster in your 50 size plane because the 12" diameter is plenty for such a small plane and it will turn alot more rpm`s with the same pitch (8) and your plane will go faster.
As a general rule OS ALWAYS recommends to large propellers for their engines, this is only because they want to keep noise down
Lug the engine down with a too large propeller and it will not produce its max horsepower and you will not be able to go as fast as you could have been.

You need to get yourself a tachometer to learn what prop sizes your engine will turn well