Wheel balance
Most the time that means it needs a front end allignment. Possible bad front tire or wheel.
I have a yz 80 2001 small wheel and does about 75 mph
could be bad alignment
Probably wheel balance.
inner tie rods bad at rack and pinion
I'm not surprised. Go to the alignment shop and get a wheel alignment. It will cost about $80 US. Unless you damaged some of the steering components. It's your ONLY option.
tires could be out of balance or or broken ply rotors might be out of round if you feel it in the steering wheel its the front tires or in the seat its the rears
80 mph = 128.7 kph
80/1.6=50 MPH
80 km/h = 49.71 mph
80 mph = 128.75 kph
The big wheel version's i would expect to hit around the 65-70 mph mark.