IDK about the big tire on the little bike for clearance, but truthfully the proper size FIM style tire for E1 size 250F( I know its an E2 machine by displacement) machines is the 120/90-18. I would use the 120/90-18 it fits for sure and will be a better choice for handling. But its your call.
I agree with Robertaccio, Its not good over tyreing a small displacement engine-kill engine response and power. It also reduces directional change ability (ie more effort to change the bikes heading). If you need a bigger foot print from time to time ie sand or mud maybe consider a tubeliss systen and let your pressure down to circa 8-10 psi
