For off-road Sur-Ron bikes you can use motocross front tyres in the size 70/100 -19 on the front and back wheels.
Street legal Sur-Ron bikes can not run front tyres on the back wheel, but there is an option, the Kenda Trakmaster K760 which is legal for front and back fitment.
Please always check your tyre sizes before ordering, we have very limited fitment information but we will try to help where possible.