Look for a nickel sized convex aluminum welch plug on the top of the engine block and/or for a 1" X 3" aluminum plate on the side of the port transom bracket.
The 1969 85HP and 115HP are built on the same motor with a slightly different engine in terms of displacement, bore and stroke. The carburetors are different.
See if the welch plug is hidden uner the paint where I have circled it. If it is still there, you could try cleaning it with acetone. The numbers will be stamped in.
Also - it's not a Johnson. The cowlings, upper & lower, have the Evinrude shape. Johnson's lower cowlings didn't adopt that shape until about 1974, at which point the uppers became interchangeable. Not likely someone swapped out the lower cowling on this one to install an Evinrude cowl...
- Scott