Dont know where to buy the wheels at the best price, but on the yz you only need the wheels.The swingarm is only 12-15 mm longer on the big wheel version.The wheel will fit,but you wont have as much room for adjusting the chain as normal,no big deal.When you buy a big wheel bike,the fork and shock springs are stiffer,so a soft spring for the big wheel version will be the spring thats normally on a small wheel version.This is off course only neccescary,if the rider is heavier.The wheels on the rm 80/85 are the same,the rear brake disc is also identical,so you shouldnt have too much trouble finding a set of wheels.
I am in the process of setting one up now. Get a copy of the 2003 Yamaha Off Road Accessories. To save some $$ I reused my hubs( just laced up to new wheels and spokes)In your owner's manual it has the spring rates for the big wheel.Cheap too.
Guys,you dont have the big wheel version in the US?We have a small wheel class,8-13 years,c and b level.Big wheels from 10 to 16 years,this is by far the most popular!We dont race 100´s here,Ive always thought our big wheel 80/85´s were 100´s with the 80/85´s engine?