If it's running slow, my experience is it's either the foot controller or the motor. The foot controller (I'm going to guess) has little carbon piles in it and they get charred, cracked and generally deteriorate. The motors are pretty straightforward. If it's not a lubrication issue (and I can't recall if those motors even have a place for oil), the brushes and commutator are what I find usually need attention.
Andy Tube on Youtube is the go-to guy for rehabbing these old machines.
Here he is working on a motor like yours:
https://www.youtube.com/watch?v=pzq4tv8qn94&t=0s
And here he is REALLY working on a motor like yours:
https://www.youtube.com/watch?v=fmEeBi8x9ZY
And here he is working on a carbon pile type foot controller:
https://www.youtube.com/watch?v=9OC79P3UWXY
Watching an Andy Tube video earns you a PHD is whatever subject he's covering. And takes about as long. But it's time well spent.