Not all race fuels are created equal. Your ATV may act a little sluggish with Sunoco 260 GT but may be very responsive and make more power with Sunoco EXO2, despite EXO2 having a higher octane. It's a matter of the makeup of the fuel. Being a naturally aspirated engine, it'll like a more volatile fuel with a lower distillation curve / boiling point and lower specific gravity. Octane is but one small fraction of the equation.
All of that said... you could probably put the cheapest E10 87 AKI pump fuel in that thing, beat the snot out of it, and it would be none the wiser. I wouldn't worry about it. On the point above, it'll like the E10 more than E-free as far as response and power is concerned because of ethanol's low BP and high heat of vaporization.