Again with the stage 2 talk...
But yeah if you have done engine work the keihin fcr 39mm carberator is a popular upgrade. Drw website sells an adapter for the motor side but you'll need alot of extra bits to make it fit and work.
If this is the route you want to go, I recommend finding a used oem carb and ordering rebuild kit(s). The keihin fcr was used on the yfz450 as well as like, a thousand other machines ranging from snowmobiles to street bikes