Engines are easy to replace/rebuild but in your position you need to examine your drive system as well. OMC is not a popular drive unit, it's difficult and spendy to get them repaired and they are easily "warped" to the point of replacement. Double your big money and it makes it a big decision as to if you want to keep the boat or junk it.
If you really like that boat, motor and drive and you are confident there's no "big" issues with the drive unit, buy a short block and trade pieces off your old motor. Anything besides that and it's how bad you want to throw money into a "hole" (aka an old boat with old equipment).
Typically the older the boat the greater the issues which mean greater costs. To do this right you should be rebuilding the starter, alternator etc. Old parts on new engines gets you by but only for a short while until the bucks start to run rampant in water pumps etc that were not replaced.