I am not all that familiar with bikes. I just happened to purchase a GS 150 two years back and its been wonderful to me on long journeys.
But I have seen people on mostly 125 and some even Honda 70. On my current trip after Eid to Kunjerab I met a boy from Lahore in Naran on a Ravi which I think is also 100 cc though I could be wrong. It was his very first trip and surprisingly enough, he recognized me from my PW thread, this very thread. He said after reading this thread he was inspired to take the trip and so he only went to Babusar Top and then came back. He came with some of his other PW friends.
He gave me his name and PW Name as well but for the light of me I can't remember it now. I am sure he will comment on this thread sooner or later and remind me who he was.
So I believe you can do it on a 100 cc so long as you have it tuned up and the right tire pressure with good oil you should have no problems.