The speedo drive on the TTR can easily be stripped out and rebuilt.
The photo below shows the parts involved and I am pretty sure the drive can't be dismantled further. If anyone else knows different please say!
I have included in the photo a brand new drive for interest.
Fit the parts in order starting from left to right i.e. plain washer, gear, tab washer, plain washer and, finally, circlip.
The replacement parts that are available are the plain washers and the tab washer. Those are usually enough to fix the drive. If all else fails new drive gears are available here.
Having stripped out the gear make sure that it is as clean inside as possible and make sure that the worm gear turns easily. Use a small electrical screwdriver or similar and use it as shown in the photo.
The photo sequence below shows the build. Sorry about the grease but I always get in a mess playing with grease and oil
Once it's all together make sure that the gear rotates freely in an anti-clockwise direction. I use a screwdriver to spin it on one of the tabs.
Replacement new tab washers in stainless steel available here.
when mine failed me having short arms and deep pockets laid tge washer on the anvil and peened (flattened ) it along the flat sections filed it to fit again and its still going .a good tempary repair untill one of the flash stainless ones is ordered or untill it fails again