If the boat was used in Long Island that's salt water but very low hours. This is a Cobra drive system, I have the same engine and you can still get most of the parts except those one piece exhaust manifolds .
If it really has not been used since 1990, here are the issues you will face.
1. The gas tank will have to be drained and cleaned out.
2. The carb will have to be rebuilt
3. Unless the engine was very well fogged the piston rings can be rusted to the cylinder walls. Don not try to crank over the engine with the starter unless the spark plugs are removed and penetrating oil is sprayed in each cyl and allowed to sit and dissolve the rust. I would do this 3-4 times over a few weeks. Then try to turn the engine by hand first.
4. The drive has to be pulled and the bellows and impeller have to be replaced, both will be dry rotted by now.
5. All fluids replaced of course.
It really depends on the climate (damp or not) and if it was fogged. Hopefully the inside did not get wet because if the boat had carpet over the plywood deck that can trap moisture and cause wood rot. I had a lot of this and replaced the deck in my boat about 8 years ago. Some of these boats had a fiberglass liner but most probably had the carpeted decks which is prone to wood rot in the core (plywood).
It is a nice boat great running and riding hull I'd try to get it going. My 88 H-200 has the same hull and its a great riding true deep vee hull, very stable and I've had it out in Long Island Sound many times.
There is a company in Canada that specializes in OMCs and sells many of the parts....
http://www.sterndrive.info/