I use a HT 2 stroke 66cc with a shifter kit from SBP.
Mandatory for high elevations
1. Rejet so your engine runs efficiently and develops max power for the elevation. MikeJ's correct about 3% loss in power for every 1k in elevation.
2. Shifter kit
a. I have a mega gear (34 tooth 1st gear). I also made my drivef sprocket 10% smaller from 10 tooth to 9 tooth and run a 48 tooth sprocket on the freewheel instead of stock 44 tooth. This combination has allowed me to climb ANY hill Colorado has to offer.
3. Tuned pipe. I tune my length for mid-range power. This allows me to have enough torque for hill climbing and enough revs for decent speed.
I've ridden my bike to 12,000 feet about 126 feet short of a peak before my shifter kit side plate distorted enough to throw the drive drain. The power was more than acceptable at 12k ASL and I had no problem pulling up the pass at greater than normal pedal speeds (about 12-15 mph).