I'd be inclined to check the carbs. No harm in stripping them down and cleaning. Definitely a job for indoors with a mug of tea. Be methodical on each carb, cleaning and reassembling, using carb cleaner where required.
Next check the spark plug gap and clean with a soft bristle copper wire brush to remove carbon deposits. Try each plug (with HT lead connected) and hold the screw thread next to the cylinder head. Get someone to pull the starter to ensure each plug is sparking.
Back to the fuel supply. Check the fuel connector is on tight. I suffered a breakdown before after my stainless steel Tohatsu fuel line connection failed with corrosion. Cleaned and lubed it up and it was sorted. Yamaha connectors are plastic and normally spot on, but check anyway. Still fuel related, drain the engine fuel filter, checking the gauze filter for any deposits.
With the air filter housing cover removed - ensure the butterfly valves open on the carbs. Again these can seize with corrosion.
Lastly, check the thermostat. If you remove it, I'd have a gasket on hand as it will tear. Use a Stanley blade to scrape the old one off. Inspect the thermostat. Any doubts, then replace. Also check the thermostat housing. I've seen salt deposits block the cooling passages before.
Good luck and keep us posted.