Seems more of a 'leaded fuel is bad mmkay' attempt at deterring the rider from using leaded, rather than a 'warning: leaded fuel will damage your engine'.

I have no idea why it would be bad, and if I knew better I'd say it really doesn't matter and leaded fuel, if anything, would be better for your motor (lubrication + anti-detonation properties of leaded). I, however know better than to advise you that it's use is okay without being better informed myself.

Unless the bike has a catalytic converter, I don't really see the harm. I doubt any two stroke bike would.
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MC21 Honda NSR250. TYGA 300 conversion.