First, welcome aboard
Second, make sure that the dizzy is the only automotive thing in the boat. for a 31 year old boat, they could have swapped an entire truck motor into it. post a picture of the motor and list the serial number of the engine (metal tag riveted to block above starter boss)
does your 1990 have an Alpha or a Bravo drive? back in 1989 and 1990, there were Alpha's as well as Bravo drives behind the 7.4's in 1990
the TB-IV and TB-V distributors were further ahead of anything automotive, plus there is the whole automotive ignition systems causing the boats to blow up thing
However GM has the Delco Voyager distributor, which for about $400 you get a distributor, coil and wires
There is also the Mallory YLM distributors and the Pertronix distribtors as well as DUI (Davis Unified Ignition) has SAE 1171 compliant versions of the old GM HEI