my old DT50 would just about manage 55 if you thrashed the living daylights out of it, but im only 9.5st ^.^, and it put up a hell of a fight and had a tendancy to go "f*ck you, ive had enough" and die until i let it settle and kept it at sensible speeds (40mph, nothing more).
im thinking maybe gearing?, smaller rear sprocket or larger front sprocket, aslong as your engine has the guts to pull it, once again, going back to my old DT (can you tell i still miss it?...) it was fairly gutless on acceleration, but was faster than a stock DT because i had the smallest available sprocket on the back and a larger front one.
failing that, fuel and exhaust systems and maybe engine management?, im unfamiliar with the 125 drag so im just shooting ideas out into the dark