I own both a 9.9hp 1975 and a 15 Hp 1976. Two important things I found on my motors, others have had different experiences. The recommended Champion spark plugs don't last. I always use the NGK B7HS for the 9.9Hp and the B6HS for the 15 Hp.
Whatever plugs you use, these motors really do not like trolling for long periods of time. They will do it but even with NGK plugs you will probably foul one of them in a day or two of trolling. I always change out the plugs every two years for sure. My motors don't get a lot of use so if I was fishing every day I would make it an annual thing for sure. Keep an extra set of plugs in the boat, but with that said, usually if you foul one, it is only one. Depending on how far from your dock you are one cylinder will usually get you home and changing plugs in the lake/river is a PITA. I always open up the throttle to WOT for 30 seconds before I bring her to the dock in hopes of burning off any carbon that might be building up.
I like to reset the points gap every 5 years or so. It tends to drift a little with use and there is a noticeable improvement in power when they are set right plus it probably helps a little in reducing carbon build up on the spark plugs.
Other then that, they are very reliable motors. If you use the right plugs you should not have any problems.