If you have a 2GM, I would contact Torresen's and get the Yanmar factory manual (AOA1025). It is specific to the GM family (1GM, 2GM, 3GM and 3HM). The aftermarket manuals are inferior.
If you have any doubt about which engine you have, they can help.
They also have complete parts lists for these engines online.
I have a 1982 2GM with a SenDure heat exchanger kit and it's a great engine if 13 HP is enough for you. We have it in a C&C 30 and it does a very fine job of getting her up to hull speed without straining. One thing that few seem to know is that it has dry cylinder sleeves, so you can simply replace them at overhaul time, avoiding the hassle of boring and selecting oversize pistons.