When I get engines in the shop that do not make full power, this is what I do to troubleshoot them, I also do it in this order to keep from making mistakes. There is a method to this and if you follow the method then you can't make mistakes, and you won't throw unneeded parts at it.
1. Compression test. Your 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. I know the engine ran well just a week ago but this really should be checked, all it takes is 1 burnt intake valve to cause the engine not to make full power.
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. Also make sure the timing is advancing when the engine is throttled up, if the timing is not advancing there is an ignition module issue. You also want to pull the distro cap off and see if there is a lot of burning or pitting or erosion on the inside of the cap or on the tip of the rotor. To much corrosion will cause the engine to misfire. To much erosion can cause cylinders not to fire at all.
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. The serial number on your engine would need to be run to see exactly how many filters it as. If any one of the filters are clogged the engine will starve for fuel.
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 boats tank, this can be a blockage, air leak in the fuel pickup and related hardware, or simply bad gas.
5. If no change, next is to hook up a fuel pressure gauge between the pump and the throttle body. Fuel pressure needs to be 30 psi always. 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, it will have to go on the scan tool. If you get to this point everything that can be checked without a scan tool has been checked. If it still does not run correctly the engine has a bad sensor on it.
If the troubleshooting is done in that order, and no steps are skipped, the root cause will ultimately be found.
Does that all make sense to you?