YT3000 Loss of Power/Speed

Welcome to the forum!

If it has a choke control, does adding choke make it better, or worse? If adding choke improves things, then you're running lean, and not getting enough fuel. If adding choke just makes it weaker and maybe produce black smoke, then it's likely not a fuel mixture problem.

If you had a video of it running, that might be helpful.

You're getting spark, since it runs, so that implies it's not an ignition-related problem. Is the gas cap exposed, so the rain could have gotten in? I guess you could have gotten water in the gas.

You can also check the new spark plug and see what the end of the plug looks like. If it's black & sooty, etc. The plug appearance can give clues into how the engine is running. This is one of many resources that describes trying to "read" your plug:
https://ngksparkplugs.com/en/resources/read-spark-plug

You have not noted the engine installed. That may be needed or not.

Anyway, most times you get flooded(drenched more likely), you get water into places that matter. I'd be removing the float bowl to see if there was water at the bottom, or open the drain(attaching bolt/ABF solenoid - engine dependent) and catch the leak out liquid so you can inspect for 'pearls' of water agglomerated at the bottom. If you have pearls, best thing is to drain everything, and make them dry before putting back water-free gasoline. The m/ethanol in the fuel will absorb some water, but you can have more.
If there's nothing there, you could still have water in the fuel tank, filter, etc. You can remove the fuel line at the carburetor and drain into a container so you can see, and again check for pearls.
It is possible water got onto the ignition, such as the magneto, and has not dried out, causing misfire. Most magnetos are under the shroud around the flywheel blower. You should remove said shroud and check for nests, clippings, and anything that would block cooling air from passing over the head and cylinder.
Finally, you could have a mess under your mowing deck. Clippings can get into the pulleys, belts, and blade brake linkage on top of the deck, and mixed with water, can make a sludge. Same as the underside, but it would likely make a ffzzzz noise as the gooey clippings were tapped by the rotating blade. You should clean out the clippings, though it likely does not reduce power, it can hinder mowing by blocking the normal flow of air from around the blade spindles to underneath the deck.
tom