Making ports bigger can just as easily make your moped less powerful. In a two cycle engine a port's location in the cylinder is analagous to when a valve opens in a four cycle engine. A port's height is analagous to how long a valve stays open in a four cycle. The angle and shape of the port is also important.
Porting engines is an entire field of engine science that I admit I know only the very basics about. I do know that if you want to experiment, you'd better have more than one engine to play with. This is definitely something where you need somebody that knows EXACTLY what they're doing before you try to modify your engine!
Be careful what you wish for! The suspension, brakes, and frame of a 50cc moped (or your Senda motorcycle) are not designed to handle high speeds. Brake fade when you've still got 50mph to lose before you cross yon busy intersection is never happy fun time!
For your reference, try these web pages:
<a href="http://www.howstuffworks.com/two-stroke.htm">Intro to Two Stroke Engines</a>
<a href="http://people.aero.und.edu/~nordlie/moped/basic_two_stroke.html">Basic Two Stroke Engine Porting</a>