I had a fast RD 125 lc back in the old days that made about 21 bhp and it would take me to 95 mph on the clock, a friend had a Cagiva Mito that was deristricted that was a bit faster on top end acceleration then he had a Gillera GFR 125 that would piss all over a Mito and an RS 125 but that was nowhere near 110 mph let alone 120.
You need about 45 bhp to do 120 mph.
A GS 500 makes about 45_50 bhp and that would only just top 120!
My TRX 850 makes 78.8 bhp on the dyno and that will only do about 135 mph.