RE's Himalayan 450s are the right bike for those roads/that trip, although we really missed the power and suspension on our KTM 950 and Yamaha Tenere 700 on some days. Some of the bikes we started the trip with had some stalling issues, but they were literally brand new. We had to switch bikes because of provincial rental monopoly issues, and those more seasoned bikes ran flawlessly. The stalling was most certainly a resolvable issue.
We each had a slight headache, me for ~18 hours and Steph for a couple of days after we got up over 11,000 ft ASL consistently. That resolved and then the only concern was if we were hiking up steep stairs or something. Elevation concerns affect different people differently, but one trip participant used a CPAP at night but seemed otherwise OK.
The RE's were entirely gutless at the highest altitudes, however.
-Anthony