motorised

I got a lowrider Springer fork for my motorized bicycle but at high speed around 20 mph it gets a really bad wobble in it what would fix that so it is more driveable

Could i put a 3 inch fat tire on the a 2 inch rim so i can put it on my beach cruiser bicycle at the rear so it looks a little bigger rhen the front?

https://www.youtube.com/channel/UCO17lH0_XM1EtcDyXn9Yofw