Well it should be a 1985 Evinrude 90,100, or 115hp or later year. That is the first year that had the anodes that hung off the ends of the stern brackets. If it is a bubble back powerhead it would be a 115. Hard to tell a 90,100 except by carb size, but there are other differences. Starting in 1986 the top HP was 110. Same basic motor as 115hp. If all the parts are original you know it is an Evinrude because of the canted motor cover. Johnson had a straight motor cover at the top corners. Like others said, if it is original powerhead the plug in the block above the port cylinder head will tell you the model number.
Dan in TN