I sell motorized comfort bicycles with a front suspension and am located in Monument Colorado. A 2 stroke frame mounted bike with a 55 tooth rear sprocket would be PERFECT to suit your needs.
Top speed in Colorado with a 55 tooth rear sprocket would be 15-20 mph depending on altitude and you would be able to climb ANY hill in Colorado. I have ridden hours and hours on mountain forestry roads on similar bikes without any problem. Contact me at email@example.com
if you have any questions.
PS To take into account loss of power with increasing altitude and increasing richness, I overcome this by adding methanol/nitromethane based fuel to the tank. I typically use 2 oz per 1/2gallon of 20/20 model airplane fuel for rides above 5,000 feet and 3 oz/per 1/2 gallon for rides above 9,000 feet. However, this is because I run a 48 or 50 tooth sprocket and don't wish to pedal on those steep mountain passes. With a 55 tooth sprocket, you wouldn't have any need to do this unless you wanted to.