I doubted whether that would be a good idea to go to Mexico or not because if you don't have passport with the visa stamp, i don't think they will let you enter U.S. The replacenent I94 is not enough to enter U.S.  You must be thinking about going to Mexico to get Visa Stamp right.. But i doubted whether they will give you another Visa or not.  Because I heard a rumor that U.S immigration has changed the policy and now onwards in order to renew the Visa, we have to go back to our own country. But dont quote me on this, i am not 100% sure. Why don't you first contact anybody at Nepali embassy, located at D.C and talk to your advisor and see what they say????