Re: Motorized bicycle setup for long distance riding
I think just about everything has been covered (and I especially agree with the "full suspension" recommendations). Running a good chain and, if equipped, the very best tensioner you can get should reduce the chances of serious chain problems. I'd also replace the axle nuts with some top quality units (if you haven't done so already).
As for visiting Yellowstone, I can promise you that this is a "must see" park (if you haven't been there already). Yellowstone is, imo, second only to (both rims) of the Grand Canyon (and I've visited just about every park and scenic place there is in the Western United States). And like they say, you see a whole lot more on a cycle than you do out of car.
Best of luck on your build and trip(s).