Around here I've never needed anything to go racing in the USA. To ride on public land in Washington and Oregon (among other states) you need an ORV tag which is $10 for 60 days I think. Doesn't hurt to have proof of ownership paperwork for re-entering Canada (if you don't have anything from when you bought your bike, stop at Canada customs before you do into the USA and get the forms) so you can prove you did not buy the bike while in the states. For the dunes you need a flag, if you're on public land (which you will be in Oregon) you'll need an ORV tag.
Have fun!