I have completely rebuilt/restored four Jeep generators and that included removing the field coils and pole faces, etc. The way I did it was: I put the generator on a homemade kind of stand to keep it from turning, etc. . I then wet cloths and put them inside against the field coils. I then used an acetylene torch to heat the area around the screw that holds the field coil pole in place. I then used an impact driver with the large bit and hammered down on the screw with the impact driver and the screws all came out relatively easy. I did try one without heating the area around the screw and I could not budge it even with the impact driver. In my case, all of the four generators were rusted up quite a bit so if your generator is not really rusty, I am sure your screws will come out easier than mine did.
The wet cloths kept the heat off the field coils and I had no problems with them or any sign of damage from heating the screws for the pole pieces.
When you re-install the field coils, you must be careful that you do not pinch the wire that connects the two coils in series since it has very little room in the case and you must make sure that if you re-wrap your field coils (like I did), that your wrap is tight and does not distort the shape of the coil. Otherwise, you will have a hard time getting the re-wrapped coils back in the genrator with no shorts or rubbing on the armature and with the field pole faces correctly seated. I had a lot of trouble with one of the generators, due to my re-wrapping changed the shape of the coils.