I have rebuilt 2 GL1200 starters. It cost about $20 for the brushes. One tip if you read the Clymer manual, the first step they list is "remove the engine". You can tear that page out if you like.
Soak your header bolts, both sides, with PB blaster. Wait an hour or a day and soak them again. Remove the negative lead to the battery. Then carefully remove the header bolts (both sides) and loosen the muffler mount bolts. The headers will then drop down so you can get at it. Remove the shift lever (now would be a good time to replace the shift lever seal if yours is leaking).

Remove the 10mm nut that holds the power cable on.

Remove the two 8mm bolts that hold the starter. The little chrome heat shield will probably fall off but if it doesn't remove it.

Slide the starter toward the front of the engine, you may have to pry gently to get it moving. Remove the starter.

Get a marker of some sort and mark both "caps" and the body so you re-assemble in the exact same position.

Put the starter on the bench and soak the long screws, top and bottom of the screws, with PB blaster. If they don't want to break free immediately, us a pair of vise grips on the screw shaft just above the threads to get them broken free.
I use a rag covering my vise jaws to support the starter in the vertical position with the gear facing down.

Remove the rear "cap" and you'll see the brushes and probably a lot of carbon dust.

There are two washer in there on the commutator shaft so don't lose them. Remove the one little screw (it looks smaller but a #2 Phillips is correct) that holds the brush with the insulation and assembly to a wire coming out of the starter body. Now lift the brush holder, brushes and all off.

Now is the part that sounds tedious but really isn't. Us a small scratch awl or a small screwdriver to clean all of the little gaps between the copper contacts on the commutator. You are not trying to make new grooves just clean them out. When finished spray some carb cleaner on the commutator contacts to clean them and blow dry.
Remove the brushes from the mounting plate. Pay attention to which goes where. One of the brushes has an insulated wire the other doesn't. The one without insulation is the one that bolts to the mounting plate. Put in the new brushes and re-assemble in the reverse order. Put a drop or two of light machine oil on the commutator shaft where it rides in the bushing on the "cap".


Bench test it and put it back it. The splined end of the starter mates with the drive sprocket but it is very easy to rotate the starter enough to get them to mate.
Put you shift lever back on and the headers and go for a ride. Then when you get back you can remember to tighten the muffler mounting bolts...
good luck and there is no magic to replacing brushes in any automotive type starter. Now if the starter is actually burned up I suggest you get a new one. We don't cover rewinding a starter in this forum.
Update 12/15/2010
Since I wrote this I have come upon, with the help of other here, a link to another Goldwing site that explains this is greater detail and has some very good photos. I have to thank Tim Wentzell for a great article and writeup. I certainly hope he doesn't mind me referencing his work here :
Part 28; Goldwing Starter Motor Refresh.
By Tim Wentzell, Mr. Magic Fingers on the goldwingfacts.com forum.
http://www.goldwingfacts.com/starterrefresh.htm