In *dry* heat I've had success with the wet thing around the neck (though I used an oversize bandana rather than a towel) and also getting my underlayer of clothes wet and then closing up all the vents in my jacket helped for 20 mins or so when things were really toasty. If your sunvisor provides UV protection then apart from a sunburnt nose the problem might be with too much airflow, over 40º you're basically blasting yourself with hot air by riding.
In humid weather I ditch the safety gear and just wear light-coloured skin-covering breathable clothes, since crashing the bike from sunstroke is a larger risk.
For both conditions I have a little cheap pop-up half-tent so I can make my own shade (and shelter from the wind) and siesta through the hottest part of the day. It weighs about 1.5Kg and takes up about the same space as a spare rolled-up t-shirt. Another general tip is that when off-roading I sit down as much as possible and conserve energy where possible ... standing on the pegs and working the bike hard generates a HUGE amount of heat.
Everyones different, some experimentation is needed to find what's best for you, just be careful because it's easy to die from overheating
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