There's a long learning curve involved here. Like posted above, the best bet is to acquire a router speed control and wire that directly in. Another possibility is a light dinner, although that wouldn't last very long because of the load on the motor causing overheating.
In any case, the only way to use a "pot"entiometer would be to replace the one in the original trigger and keeping all other components. In that case, you must be careful to use a "linear" taper pot, avoiding at all costs an "audio" taper pot. The audio taper will measure electrically the same as a linear taper. The problem arises when you want control "here", the control is actually "over there" somewhere.
The component you are looking for/into is called a "rheostat". It is similar to a potentioneter electrically, but rated for a much higher current, 50:1 or larger. The cost of a rheostat so far exceeds that of a router controller that the router control becomes the only practical option. There are other options but you must watch for full 120 volt operation. Many such controllers are rated for 40 volts or less. And D-C output. Such a controller must be avoided
.