[Al Skierkiewicz]

komeiji,
***Note: Under current rules, this is not a legal modification of this motor!***

The method you would use to open these motors is first remove the transmission and adapter plate. Then I use a wire cutter to pull back the outer case at each point it has been crimped over the end plate. Using a long, slender tool, i.e. screwdriver, push gently on the gear and shaft to push the end plate and armature out of the case and magnet assy. Once you have The armature and brush assy open, you can gently move the brushes to release the armature. Inspect the armature to see if the wires are burned or are dark in color. If so, dispose of the motor it is not repairable. If everything looks normal, set the armature where it will not roll off the table. Look at the endplate with the brush assy. (The picture in the linked thread will help.)
You will note an axial leaded component that resembles a coated resistor. This is an inductor that is likely burned open. If you carefully unsolder this component, replace it with a short wire. Assemble the endplate and armature being careful to avoid damage to the brush assy. Then using a long slender tool, reinsert the endplate/armature assy back into the case. Try connecting a battery to the motor and see if it spins. If it appears to be working correctly, then using a hammer and some other tool, recrimp the case to hold the endplate in place. This will give you a working motor that is not legal for competiton.

***Note: Under current rules, this is not a legal modification of this motor!***