I would put a spark tester and confirm adequate spark however seeing it sputters i would be ok with that for now. Next I would deal with the fuel delivery side and you can take the fuel line off the carb and see that fuel will gravity feed out the hose at an adequate level as that will rule out any filter or hose blocking/deterioration concerns. I understand you say you have clean fuel but change it out new fuel, will eliminate bad gas or water in the gas. With those things you should have spark, fuel and fuel delivery from tank so now onto the more likely concern of fuel delivery into the engine in cleaning the carburetor itself, from gummed up & blocked passages, gummed up needle valve, clogged jets, water droplets in the bowl….either way a thorough cleaning will go a long way here. Carb cleaner down the throat of the carb is always good to see if it will sputter which you have confirmed it did so that is why fuel related is my first bet. Once that is done we can go onto other items like timing, valve adjustment, etc however spark, fuel deliver, compression, timing are things to hon into.
Plenty of more experienced gentlemen will put forth their thoughts shortly.
Cheers.