If you have a motor that the wires are good but the insulation is brittle and cracking, or just damaged, all is not lost. This is a relatively easy way to fix the motor leads.
I take no responsibility for this, but it is how I fix the wire insulation on these motors.
Overview. What you are doing is removing the motor, opening it up and pulling the wires out of the motor case, breaking off the old insulation, twisting the wires to make them neat and tight for new insulation and then cutting them so you can get new insulation on the wires by using heat shrink tubing.
First open the case up and expose the wires in the case. Remove the bad wire insulation by cracking it off, preferably by hand. (extreme left in this picture)
Twist the entire wire clockwise to get them tight so you can cut them about 1.5" away from the wire end if you are going to reuse them as I did in this example. I took a picture of the wiring so when I put it back together I know where things go. (Take the picture. Trust me, it might come in handy later.) Notice the bad insulation.
I use two sizes of Heat Shrink Tubing so I can have at least two layers of insulation on the wires. I use 1/8" and 3/32" tubing.
I "Tin" the ends of the wires with Electrical Solder and I always use Flux.
Tin both of the ends to be connected. In this case 2 wires, 4 tinned ends.
Slide the heat shrink tubing onto the wires BEFORE you solder them together. Otherwise you will not be able to slide it over the looped end. Note in the picture you can see two unshrunk tubes on the long length of wire.
With both ends of the wire tinned, heat them and overlap them just a bit so you don't get a huge bulge of wire and solder, but overlap enough to give it strength.
Once you have them soldered, slide the tubing over the bare wire you soldered and heat it to shrink it to the wire while butting the ends of insulation tight to each other for a good seal. Do not overlap them at this point if you are putting another layer over this one.
Be careful to not shrink the other tube while you are doing this step.
Slide the second layer over the seams of the first layer and shrink them. You should never have two seams on top of each other, but covering each other.
I used a crochet needle to grab the looped end and pull a third layer over this second one to provide further insulation and make it all look neat and tidy. I did not show this step however. It wasn't easy and it was not totally necessary either but that extra thickness of a third layer is nice insurance. But I feel that two layers are adequate.
Clean and put a drop of oil on the shaft, check the brushes and clean inside the case, clean the armature, and put it back together as it came apart.
I hope this helps.