Axle size , brake rotor size , wheel spacers are just the big ones . The weight savings is hardly worth the price of the Used parts , and a Home depot axle nut from el chapo or what ever his FB name is .
Now that said if you have left over good used parts or access to free stuff it can be worth it.
However it will never save the weight of after market forged or carbon wheels .
Yamaha has the lightest factory stock wheels of all brands due to their casting technology .
R1 , R6 , FJR1300 all wear these with dished out spokes in the wheel outer tire face saving weight on the furthest points out .
Start saving your pennies , and look out for sales or a used set of aftermarket wheels on e bay or forums .
Many times owners will sell off the high dollar aftermarket stuff when a 10 year old bike won't bring top dollar on resale .
3 years ago a set of Gen 1 BST wheels with the rear being brand new sold here for $1600 shipped .
I turned my friend onto them , and he purchased them. Arrived like described , and zero issues mounting them up .