What is the value of a 1986 90HP Mercury?

That is a hard answer to give. Without looking at the engine and checking compression there is no real way to give you an answer. That engine was great when it was running properly, but it had a few issues. A lot of factors come into play such as was the engine used in salt water or fresh? Also to consider is most people today want 4-stroke technology, not the old carburated 2-strokes. IMO I feel the old engines such as yours were the best made, simple, less moving parts. The best way to gauge the price is to check on eBay and see what similar engines have sold for.