Dan,
Sounds like you'll need a bearing plate as well, and I'm thinking that wherever you find the armature, they will have a bearing plate as well.
As for the teardown, first remove the belt, then the motor brushes. You'll find caps on both sides of the motor that unscrew and the brushes will be behind the caps. Once you have removed the brush caps you can remove the brushes by gently pulling the springs.
Whether your model has a fixed headlight or swing-up headlight (or no headlight) will determine the next step. If you have the fixed headlight, remove the two screws that attach it to the fancase, then turn the machine over, remove the brushroll, and remove the two screws that attach the light to the nozzle.
If you have the swing-up headlight or no headlight, skip the headlight removal step and go directly to the next step, removal of the fan case and nozzle. You will find 4 screws that attach the motor housing to the fan case, remove these and the fan case and nozzle can easily be removed.
Since your armature shaft broke behind the fan, I'll skip the part about how to remove the shaft and fan. Sounds like they'll probably fall out when you remove the fan case. You'll find 3 or 4 screws on the bearing plate, and after removing these and the bearing plate the armature should easily pull out. However, if the fan and the belt shaft are in good shape you'll probably want to save and re-use them, so put the fan in a vice or hold with vice grips and turn the belt shaft CLOCKWISE to remove. Hopefully you have a big enough piece of the motor shaft behind the fan to get a hold of with vice grips, as you'll also need to turn the fan CLOCKWISE to get it off the motor shaft.
Once you have your new armature and bearing plate, make sure the armature seats well into the rear bearing, then install the bearing plate. Snug, but do not tighten the screws. Note here that there are two types of bearing plates--perma-lube and oil tube. If it is an oil-tube bearing plate, add oil until it reaches the top of the tube, then turn the shaft to make sure it turns freely, then check the tube to see if you need to add a little more oil.
Next, re-install the motor brushes (or replace if they are less that 1/4 inch long) and caps.
Now, tune up the motor. Plug it in, turn it on, and one at a time slowly turn each of the bearing screws in and out to get the highest speed (kind of like the carburetor on a lawnmower!) Once this is done, reinstall the fan, fan shaft, fan case, etc., and it's done!
Hope this helps,
Jeff
