The shop I took ours to does a fair amount of marine work plus starters/alternators in general. He said he couldn't get a drive for ours. But then, all it was, was stuck. We'd come back from being away a few weeks in a very rainy season. I think the drive just stuck to the shaft with a little surface rust. I canceled a patrol, pulled the starter, fiddled with it and the drive would extend, every time I tried it. At that point I took it to this auto electric shop along with two RV starters. He "serviced" the three for $30 each. Good deal on the Ford RV starters. They got brushes, bushings, and as I recall one drive. The MerC starter was really just Clean, Inspect, Lube, Reassemble. It came in working and went home working. Has worked for years since.
I liked working on the Delco starters because so much was replaceable without special equipment. The Ford ones, and Imports, required Spot Welding to replace Brushes.
I bought a spare starter online, forget how I found it. Outfit in Milledgeville GA. Also a Delco Alternator as a spare, some online place in FL near Orlando. Neither has been on the boat. 1,339 engine hours as of this morning. Up at 0330, underway 0500 for a fishing tournament start. Home 0900 before it got hot. Beautiful day!