The hassle with a decades long Keys dock queen would be the REST of the installed systems.
Spend the time to catalog all the equipment and be sure its suitable for your desires , and see it all operate.
Swoping a dock queen outfitting to what is used on a cruiser can be costly and time consuming.
The engine could have been started once a month for1/2 hour (not good) or just ignored , never pickled or even serviced.
PH rot and deck rot would be a bigger fear than an old fuel tank.
House boats are always a big question , unless you will continue as a house boat.