Keep the carbs and have them rebuilt/tuned and add electric chokes. Install HEI ignition distributors. I have run Qjet carbs with DUI (Davis Unified Ignition) distributors for 9 years with no issues.
I suspect it's an add on but potentially dangerous one. If the 2 outside receptacles are connected to shore power check to see if the cockpit receptacle is live. If it is someone could get electrocuted by the exposed terminals. Doesn't matter if there is a hinged cover on them.
Of course timing could be an issue but start by checking the condition of the distributor cap and rotor. If moisture is present it will cause ignition cross over and cause those symptoms.
One way for you to determine if the engine is reverse rotation is to check the location of the plug wires on top of the distributor. Compare to the port engine. My guess is by 1992 both engines are standard rotation and one has a reverse rotation outdrive.
At my yacht club the majority of generator problems are with Kohlers. With all makes the most problems are with newer boats that have computer controlled integrated generators that tie into the carbon manoxide detectors. The older simpler generators seem to have very few problems.
I bought a my 37 year old Sea Ray 13 years ago. It had the old style Tides Strong Seals in it when I bought the boat. !3 years later they have yet to leak a drop. I don't understand why people have trouble getting dripless seal to work properly.
Rebuild the carbs and reseal the well plugs when rebuilding. Convert to electric chokes and be patient getting them set correctly. I did this to my 454s along with Racor fuel filters and even after sitting all winter they start without pumping the throttles. They will crank for 3-5 seconds but I...
It may be easier to take the entire engine transmission assembly out to gain easier access to the components. While it's out address other pending issues such as raw water pump, belts, alternator, manifolds, risers, plugs, etc. While your at it check the oil pan for rust/corrosion as the shaft...
Those symptoms indicate the forward clutch packs have fused together and will not disengage. Did you perform a high rpm shift from reverse or neutral into forward?