Like everyone says, start with simple things. Don’t replace expensive items like a power pack on a hunch… You could have any of the problems you mentioned… Let’s begin with when was the last time the engine ran OK. Has it been sitting for awhile? Is the fuel/oil supply fresh and mixed at 50:1?
If you suspect a fuel pump/delivery issue, there is a simple way to check this out. Simply pump the fuel bulb on the fuel line while the engine is misbehaving, does the RPM pick up? If not, fuel pump is probably not an issue. If the RPM does pick up, well then you have some sort of fuel delivery issue… Perhaps the fuel tank vent is plugged/closed, fuel tank pick up plugged, air leak in fuel lines/connectors. Or, perhaps there is a restriction somewhere in the inlet fuel line. Is the fuel pump screen clean?
You mention the engine “cuts out” when you try to advance the throttle…. Do you mean it bogs down, or actually misfires like it is losing a cylinder… Any way you could post a video of the engine running/misbehaving? Perhaps the carburetor high speed jet is dirty, or the carb has gunk in it. Again, when was the last time the engine was used? Sometimes, you can carefully partially close the choke when the engine misbehaves to richen the mixture up a bit. Maybe tough to do on your engine, kind of depends on how the choke linkage is set up. Nonetheless, if partially closing the choke picks up the RPM briefly, then most likely you have a dirty high speed jet/carb.
The engine could be misfiring at higher RPM, have you replaced the plugs? Are you sure it is running on two cylinders at lower speeds? Doing a spark test probably won’t show much only because you report the engine runs fine at low RPM, so the spark check will probably show good fire on both cylinders. Those inexpensive neon inline ignition testers can help isolate an ignition issue at higher RPM. But, please be careful running the boat while trying to watch the testers, don’t get hurt. Usually best to have an assistant along and do this test while the sun is going down so you can see the neon testers…
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