I was hoping you would reply back but I never heard back from you, so I will leave you with this.
The engine needs 3 things to both run and make full power. What it needs is compression, timed spark ignition (spark happening at the right time) and the proper ratio of fuel to air.
What I get engines in that have the same symptoms of yours what I start with is engine compression and spark tests.
1. Compression test. Results need be 100 psi or better, all cylinders should be within 10-15% of eachother. If any cylinder is under 100 psi it is either worn out, or has a valve problem, and the heads will need to come off and the valves inspected next. A healthy engine will read in the 140 to 160 psi range.
2. Spark test. The easiest thing to do here is to hook up a timing light to each plug wire one at a time and see if the light flashes or not. Flash means spark, no flash means no spark. No spark can be something as simple as a bad spark plug, that was dropped on the floor before you got it, bad plug wire, or a bad cap. As part of this test the timing advance needs to be checked to make sure the timing is advancing when the engine is throttled up. If the timing is not advancing the engine will not make full power. If timing is not advancing, the springs for the points are frozen up. The timing should read roughly 8 degrees at idle, and shoot to 25+ when you snap the throttle open. You also want to mist the plug wires with some water from a spray bottle, if the engine starts to misfire when the plug wires are wet, the wires are definitely no good. Lastly if it looks like spark is cutting out you want to replace the points and condenser again. Also, when you set the points, make sure you do them the proper way by setting the gap first, and the dwell second. If the dwell is not properly set the points can burn out prematurely. If the dwell is not set, the timing will not be correct as well.
3. If compression and spark are good, make sure the fuel filters are all changed. Some engines will have up to 3 fuel filters on them. With 1 large canister filter in the front, followed by paper filters in the carb and fuel pump.
4. Next is to get a small outboard tank (like a 3 gallon) and fill it up with known new fresh gas and try it. If the engine runs better on the portable tank there is an issue with the fuel in the tank or with boats tank and plumbing. You would check to see if any of the fuel lines have collapsed, check to see if the tank has an antisiphon valve on in it (and replace it if there is one) and if necessary, pump out the tank, pull the fuel sender, and scope the tank to look for trash in it.
5. If no change, next is to hook up a fuel pressure gauge between the pump and carb. Fuel pressure needs to be in the 3 to 7 psi ballpark at max speed. If it's not, the fuel pump is weak and needs to be changed out.
6. Lastly, if you got this far and the engine STILL is not running correctly, everything else has been ruled out at this point, and the problem is inside of the carb. At this point carb will need to be either rebuild, or replaced (depending on how nasty it looks inside)
If the troubleshooting is done in that order, and no steps are skipped, the root cause will ultimately be found. Also keep in mind there is always a chance there is more than 1 thing wrong with the engine. So if for example if you pull apart the fuel pump and change the filter inside of it, and it looks nasty but the boat still does not run correctly, try not to get discouraged. The older the boat is the odds of it having more than one problem goes up. I couldn't tell you how many engines I have worked on that have come in and the boat has bad gas in it, all the filters are trashed, the ignition system needs a tune up, and the carb is clogged as well. It definitely does happen.