Big Bore kit only giving 26bhp. 4-6 bhp over stock? Might be enough with the extra torque to cruise a 70mph.
I remember following 2 x crf250l with luggage riding down the A38, at 60 to 65 mph the engine was screaming.
I am able to cruise my Himmy at 70mph. and, have done many times without a big bore kit.