Before electronics, we used a rheostat to change voltage going to the router. I'm not much for reworking tools like this, so for me,it would be time to replace the router with a better one, and that for me would be a Bosch 1617. Or, if I used it in the table, I'd get what I have now, a Triton TRA001. Too heavy for hand held use for an old guy like me. But fantastic in the table and you can adjust height from above the table by simply removing the spring from a tube. Lots of good safety features too.
For hand held, I'd get one of the smaller, compact routers. The only limit is they only take a 1/4 inch shank bit, not the half inch of the Bosch 1617 I have two 1617s I set aside when I bought the Triton, but I really don't use them anymore. But they were great when that was all I had. One for the table, the other for the fixed or plunge base. Now I have a motor for each base. One of these days, I think I'll sell the Bosches. I have lots of great accessories for them too.
That's my take. I think you can find a rheostat voltage controller on Amazon. I lost track of the one I had for my first not-so-great Skill routers. Might have given it to the guy who bought the router. Anyway, I got it on Amazon, but you might find on at Harbor Freight. Haven't been in there for some time.