Have you checked the compression?
Electrical is usually my first go to for an engine that runs poorly hot, as those components are most sensitive to temperature. Start by checking spark when it's hot, should be a bright blue. White to yellow is pretty weak, and will start to result in misfires. From there, you can check individual coils by measuring their resistance and comparing to spec.
After that, look at fuel mix. Note the color of the plugs when you check for spark, chocolate brown to a cardboard tan is ideal. If the engine's breathing warm air from under the cowl, it could be causing it to run rich as it gets hot. I've also had an exhaust restriction cause a rich condition when hot, and only in temperatures over 25°F. That one gave me pretty mild symptoms though, it'd lose a few hundred RPM a few seconds after hitting my target shift RPM at WOT, but otherwise had good performance and response.
And, just to be clear, it's only a hot issue right? If it's just a general "revs fine on a stand, but not under load" then you'll want to start by cleaning carbs, then cleaning and inspecting the clutches.