Father bought property, didn't register it, not alive anymore.

My father bought a residential plot in 1996 in Jaipur. But he never got the the property or sale registered. Our family realised this when searching for the documents for my mother to inherit the property, after my father's demise recently. In the name of documents only thing that we have is the sales document on the Housing Committee's(that sold the land) own letterhead/certificate kind of thing, with signatures of all the parties and a little map with location/dimensions of the plot attached. What would be the advisable course of action to get the property registered in my mother's name? Should we get it registered or should we sell it to next person(if that is possible?) ? And what kind of problem we will be facing in getting it registered? Is stamp duty to be paid at current market rate or at the transaction rate when my father bought it? A little heads up on the issues we will be facing would be appreciated much so that we can think and plan beforehand, as we are currently living in Delhi and planning to go to Jaipur for a couple of days to get the registration done. Thank You in advance.