Do you want a 20" tall skinny tire or a 24" tall fat tire? You will need a fat tire fork, which is 135mm wide at the axle, in order to fit a motor with a fat tire. The typical front fork for a normal tire is 100mm wide.
I'm going to have to back up and wait for you to answer Timpo's question. THe rims for the above wheels are exactly 16.0 inches in diameter although widths differ. If your wheel is really 16.5", it would have to be a 14" wheel. Anyway your current tire will have the tire size on it.