Most s-2 9.1's had Yanmars in them. Don't know if your boat is in the water or out, but usually a part of the winterizing process is an oil change. If your boat is in the water, start the engine and let it warm up. It is easier to extract the oil. With a pump (hand or electric, or other fancy pumps available for getting oil out of your engine) pull the oil out through the dip stick hole. Remove the old filter and replace with a new one (after filling it up half way with the designated oil - look in your owner's manual - probably 30 wt).
You can still change the oil after the boat is out of the water, but it might be harder to extract. When done, then fill up the oil to the proper level on the dipstick. Each boat can be different even with same engine depending on the angle it is installed.
To winterize the cooling system, you probably have a raw water system and a closed water cooling system. You can test the antifreeze in the reservoir to make sure it is topped off and it is the proper mixture (50:50) to protect it when it freezes outside. Regarding the raw water system, where the cooling water is pulled out of the lake or sea, you can run a proper mixture of antifreeze through the system. If your boat is in the water, close the intake seacock that supplies sea or lake water to your engine. If your boat is out of the water, you don't have to worry about the seacock being closed as you won't sink on the hard.
Pull the hose off of the seacock (usually 3/4") and stick it in a bucket of mixed (or premixed antifreeze for engines - non toxic kind is preferred). Or, if it is easier to, with the help of another person, pour the antifreeze into a funnel stuck in the hose, that is ok too. Make sure that the antifreeze is always present because the next step is to start the engine and let the antifreeze flow through the raw water system until it comes out of the exhaust.
If you have a hot water heater with a heat exchanger, this will be a part of the closed cooling system.
After you see a good flow of green coming out of the exhaust, you can shut the engine down and reattach the hose.
You probably have an aqualift muffler (can look like a can or if Vetus type, a brown plastic container). This keeps sea water from backing into your engine. Usually there is a plug where you can drain the water from it.
You can also remove the rubber water pump impeller and store it out of the water pump coated in a water proof grease. Put it back in the spring. This will also drain any liquid out of the pump housing.
Before you pull the boat it is recommended that you fill your fuel tank and put the proper amount of StaBil fuel conditioner (for diesel) for the amount of gallons in your tank. This will keep the fuel stabilized over the winter and the full tank will eliminate any condensation from forming.
That is about the basics for winterizing the engine. If you do at least what I have mentioned above, then you should be safe for the winter.
There are other things you can do, but to protect the engine from freezing, this is the least you can do.
Of course, if you have a fresh water system and a marine toilet, you need to winterize those as well. But use a non toxic (red) antifreeze and not antifreeze for engines.
