My boat came with a comprehensive model specific manual complete with schematics, so they are out there.As Minnewaska mentioned, most manuals with a boat are for the engine, electronics, etc and are specific to the manufacturer of that equipment. Never seen a manual for the boat itself other than a glorified sales brochure. Maybe if you could elaborate on exactly what kind of changes you're considering?
Unfortunately on a boat built in the '80's, if there was ever such a manual it is likely lost forever. There is a remote chance that someone else with the same boat managed to hang on to theirs, but it is a long shot!
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