Re: 40 hp johnson lower end repairs
RDS22 is the model number, which correlates to a 1960 Johnson 40hp. It's a mechanical shift and, unless it's been converted to a long shaft, it fits a 15" transom.
Lots of threads on that great old engine.
If it isn't going into gear, then the first place to look is the shift rod connector, which you'll find behind an oblong shaped cover on the midsection. You'll need to remove the outer covers to get access to it and to the connector.
The pin being referred to could be the drive pin for the prop. If you remove the cotter key in the plastic cone on the prop, you can see the drive pin. It goes through a brass sleeve that extends from the front of the prop and through the prop shaft. It's available from any Evinrude dealer, and fits outboards from 1958-1968 and beyond into the 1970s. If that's bad, it will prevent the prop from turning under power. It's a cheap thing to replace and you should have a couple of spares on hand.
You definitely need a manual, and I don't know of a free one. You can, if you must, get by with the SELOC manual that covers a bunch of engines, but the factory manual is best. Sadly, your engine is covered only in the Master Manual, published in 1964, which covers all of the one and two cylinder Johnsons from the 20s through 1964. Still, it's the best manual, even though it costs a good deal.