Keep in mind that the windings on a motor are setup for a specific range of RPM. Most electric bike motor manufacturers don't just limit the motor to 15-20MPH, they build the motor to travel at this speed at full voltage (this also means the motor is performing near peak efficiency at these speeds too). You can make the motor go much faster by boosting the voltage, but then you will need a new speed controller, and will run a very real risk of frying the motor during an acceleration or while climbing a hill.
Another idea for the knees would be to take your bike to a local bike shop and get the bike properly fit. Also, if you pedal at a slow cadence, this can also hurt your knees pretty fast, so a lower gear and a higher cadence will help a lot. Knee pain can also be from having the wrong size,shape pedal for you feet (I've definitely had this happen to me). All in all, a local bike shop should be able to look at your fit on the frame, as well as your riding style, and figure out where the pain is coming from and how to fix it.
Ok it looks like i have to except 15 M P H i have already changed the chainring from 52T to 48T and this has helped a bit if i could turn back the clock back i would buy a light weight 21gear bike and put a E kit on it.