Welcome. I found mercury manuals online (a Seadoo/Merc FB group offers them in its file section). Quicksilver 2-stroke oil is good, I use it (same engine), there are other favs out there too. I would change spark plugs, run a compression test (all cylinders) to know what you got. Most will recommend to remove the oil injection pump and buy the block-off kit and go premix (sadly common issue with plastic crank drive gear for oil pump kills a lot of these engines). I also would go through the jet pump and change stator and gear oils and inspect for excessive wear and/or bent impeller/stator fins. Lube your steering cable. Lube your throttle/shift cables. I would rebuild the pulse fuel pump (cheap rebuild kits). I would change the fuel/water-separator filter. I would remove and clean the lift-pump prefilter (small filter on the fuel line that screws into the lift pump (the clickity-clack pump). I would prime fuel system and ensure the VST (port-side tank that hold the high-pressure fuel pump) is getting sufficient fuel (manual explains test). I would also prime fuel lines and ensure getting ~34psi fuel pressure to the injectors (there is a schrader valve on top the VST to test). Ensure you got fresh fuel (I prefer E0 fuel). Then take it for a water shake-down cruise and see if there are any other issues. GL