I don't know about 1990 but the current ones are three cylinders. Easiest way to tell is take off the cover and count the number of spark plugs. As far as manuals go try the manufactures web site (if their is one) a lot of them have manuals or basic information online.
Undo the cover on the motor and tilt it up, then count the spark plugs, if mariner doesnt have a web site mercury is the parent company and my marina says that they are virtually the same motor. Or you can go to a marine parts place and ask for plugs for your motor, they can look it up for you.
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