I would recommend any Royal Enfield motorbikes.
Because I have been there and I know.

Hello Umesh,

I have been to Jomsom-Muktinath 3 years back, I think Avenger will be ok, else you can choose some dirt bike to travel because the roads are rough and zigjac. I went there with the local bus. I saw few petrol pump station in pokhara, in beni but from beni to jomsom i didnt saw any. There are few stoppages in beni, ghasa, tatopani and jomsom where you can rest and enjoy the local meal. I think you should carry extra petrol for the backup.

The total cost of the journey was Approx. Rs.10,000 per person including hotel stay, transportation service and others.

If you are still confused with my answer, you can jump on this following link with some experienced people's reply :
1. http://www.yentra.com/question/20 (Traveling to Jomsom-Muktinath)
2. http://www.yentra.com/question/5 (How Long is the drive from Pokhara to Jomsom?)
3. http://www.yentra.com/question/94 (Kathmandu- Jomsom- Muktinath Blog (23 Photos))

Enjoy your trip and have a safe journey.