Impeller / Water Pump should be replaced every year or two. Does not include impeller plate which does not need to be replaced every time.
Sierra International 18-5156 Contact Set
Sierra Marine Replacement Ignition Coil. You need one coil for each cylinder. Old coils often become cracked or disintegrate over time because of the epoxy sealer originally used. New coils made with different materials are much more reliable than the original.
Hard to find but people are selling them on eBay.
Difficult to find. Other option is to make your own gasket. Packing nuts are same as other OMC carbs.
Available at VintageOutboards.com
You will need one of these for each cylinder.
Difficult to find. Not available on Amazon.com.