Check the field coils first.
If they have continuity and are not shorted to the case-reuse them as is.
Dump the core in the tank and scrub out all the stuff,blow it out real good ,then RECHECK AFTER THE BATH.
As long as the fields are not grounded to the case they are keepers.Shorted fields can even be saved by rewrapping the shorted coil with surgical suture tape.
Obviously you need a new armature.And the new brushs will have to be seated in with a soapstone.
Give it a whirl.Hell ya got it home-that was the hard part 
If you really have to replace the coils,a blunt punch will suffice to unscrew those square screws.Or an impact driver.Personally I would rather use the punch method.If you "punch" make sure to grind down the punch knubs caused by the punch impact as they will cut you.
I just reread the post by marvh.I bet we worked in the same shop!
Oldmics