Better check where you're going before modding it out. If you're actually going to be *in* Yellowstone, I don't believe you can even bring your own sled into the park anymore. The tree-huggers have pretty much shut that all down. Not saying there aren't areas you can ride without being part of a "tour" group, but I'd make sure about it before heading out there.
Of course, you're always welcome to come to the "greatest snow on earth" here in Utah. We have some of the best riding anywhere in the US here, and it's not overly restricted (yet).
2004 Polaris Trail RMK 136
1997 Ski Doo Summit 500
2000 Arctic Cat ZL550 ESR (in NY)
2000 Arctic Cat 440 Panther (sold)
1980 Arctic Cat Jag 3000 F/C (in NY)
"If it ain't broke, take it apart and find out why!"