Looks like the same unit I got from TRF back in 2000. Worked well for about 10 years, then started becoming intermittent. Just every once in awhile, wouldn't crank at all. When I pulled it apart to look for any problems, I found that the bearings that carry the motor armature were ruined. Apparently the heat from the nearby exhaust pipe had cooked all the grease out of them. (That bulge with the gold cap in your photo is the actual motor. When installed on the car, it hangs down so it is actually much closer to the exhaust than the stock starter is.)
But it was easy enough to replace the bearings, and it still works today. I fashioned a very crude heat shield out of some roofing tin I had handy, which might possibly help the new bearings last longer.
The actual starter motor is a very common Japanese starter (designed by Hitachi but often cloned by others), so any "auto electric" shop should be able to change bearings if you don't want to do it yourself.
Oh yeah, I don't think the bearings were the cause of the "no crank" problem. I also discovered that the quick connect for the solenoid was just a tiny bit loose on the tab, so I think it was sometimes not making good contact.
One other thought : I didn't want the main current for the new starter going through the original solenoid (which had lost some of it's smoke), but I also didn't want to lose the rubber button on the original solenoid. So I rearranged the heavy cables at the original solenoid so the new solenoid on the starter gets power all the time, and ran a new wire from the old solenoid output to control the new solenoid. I'm very happy with that setup. Here's a shot where you can kind of see how I moved the starter cable up to the top post, and the new brown wire to the new solenoid from the bottom post.
