Sorry, I don't have any particular knowledge/insights to help too much with your question.
But am curious what is the background that gives you pause for needing a spare starter? I have a 2QM20. Wouldn't be surprised if it has the exact same starter as yours. A couple of years ago, I came across a website that claimed to have a Hitachi marine starter that corresponded to the one in my 2QM20 and at the same sort of price that you cited for the one you have seen. I was tempted to order, but my starter had never shown signs of acting up. For the heck of it, I removed it from the engine, took it home, took it apart, cleaned it up and checked according to the inspection/tests suggested in the Yanmar manual. The brushes were getting near the minimum acceptable limit, but otherwise all was OK so I stuck it back on my engine.
But relying on a 30-year old starter does give me some concern, particularly since the starter solenoid is intregral and can't be serviced (such as cleaning the contacts) because it is a sealed unit.
Without a working starter, the engine is useless. I've tried several times to start the engine with the manual emergency crank ... with not a hint of success. The resistance of the cylinder compression is too great to be able to get enough rotational speed. And for one person, releasing the decompression mechanism to get some rotational momentum, and then engaging again, at the same time as trying to crank, take too much coordination to accomplish.