Dax Titan engines running in Utah mountains http://www.youtube.com/watch?v=y3M048cSJYU
A 2000 mile trip in 20 days would be 100 miles in a day. I have ridden bicycles and motor bicycles up to 100mi in a day and it can be a real chore, esp in really hot or windy weather, and if you have any breakdowns it can be very stressful.
With most motor bicycles, you get only one motor speed/gear. If it is low enough to climb mountain grades without exerting yourself much as a bicyclist, it will be slow and consume a lot of fuel.
Whatever you do, if you are going a long way on a bicycle, it is good not to travel alone. Best of luck!
I am a fan of Dax Friction Drives. With a 1" roller, and a 50cc engine, you should be able to do 20mph on the flats, and it should be able to grind up most grades with minimal input. You will be able to conserve fuel by idling or coasting down major grades. The 1" roller does not freewheel, and can be used to control descending speeds, or can be lifted off the wheel to accomodate the highest speeds or pedaling without the engine running.