They seat completely differently, the Wood Elves are positioned to imitate being in stirups, and the Bikers are more posed as if they are driving a car.
However by cutting the legs off the Elves and re-positioning them then cleaning up the gaps with some Green Stuff you can probably achieve it, I would use the Games Workshop 360* of the Windrider Jetbikes as a reference for setting the legs, it should be easy enough but not an easy swap of riders.
GL
Edit:- More like driving an F1 Car before anyone calls me out on that