I had the same issue with my Diamante starter. The cam thrust case oil leak runs down and gets into the starter. I probably could have cleaned the starter and replaced the brushes (available separately from
http://www.parts.com), but I went with a remanufactured one from AutoZone for about half the price of an OEM Mitsubishi replacement from parts.com. By the way, I found that Pep Boys, AutoZone and Advance Auto all sell the same remanufactured starter, just under different names.
As to the cam thrust case oil leak, I sealed it with silicone. See this thread:
http://mitsubishiforum.com/forum/showthread.php?t=34354. I also sealed the seams on the new starter with the same silicone just in case oil gets down there. The starter remanufacturer said sealing the starter was okay to do.