spark, compression and fuel. I would just do it all without trying to outguess things. Check what is easiest first and rule out issues -- 1/2 inch spark with a spark tester, compression test (already found all 3 close to the same, so apparently OK there), drop cylinder test (confirm all three firing). If no issues found, completely disassemble, clean and rebuild carbs. Make sure the fuel supply is fresh and clean with no water before testing with clean carbs, or you may end up having to repeat the cleaning. Permanent tanks in a new-to-you boat are a particular concern for old or contaminated fuel.
If you are over choking/over priming by mistake, you may be running too rich, and stalling -- likewise if there's too much oil mistakenly supplied through a combination of premix and injection. You will see some fouling in those situations -- not fatal to the engine, but won't run well until corrected. And on the flip side, if the pump diaphragm has a tear or hole, the fuel supply won't be sufficient and the motor could stall out when throttling up.
As the motor is new to you, the impeller needs to be replaced. You need to be sure it's not overheating.
So what is the model number? A lot of listers are very familiar with the 70s, and some good pics without the cover on could be useful -- with and without the cover, for that matter.