How to register flat if the owner is deceased and builder has closed the business.

My father had purchased a flat from builder in 1983. As per the then practice the flat purchase agreement between builder and my father was on Rs. 5/- stamp paper and it was not registered. My father expired in 2008. Builder has closed his business in 1990s and his whereabout is not known. Original agreement papers are missing. We obtained a xerox copy of agreement between builder and my father from society record. My father had not made his WILL. Legal heirs of my father are my mother (age 80), my younger brother (age 49) and myself (age 53). This flat was transferred in the name of our mother in 2008 with our mutual consent (i.e. by my younger brother and myself). All the society's maintenance bills and receipts are in the name of my mother. Share certificate of the flat is transferred in the name of my mother. On society's members name board / List, name of my mother appears as the owner of the flat. Electricity bill from MSEB (Electricity board) of this flat generates in the name of my mother. In the backdrop of above, Our questions are:- 1. What is the way out to register this flat in our mother's name. 2. What document or legal procedure would ensure or strengthen our ownership of this flat in the absence of original documents and non registration of this flat. 3. If this building go for redevelopment what documents builder will ask from us. 4. Do you foresee any hindrance from new builder when our building will go for redevelopment or can he take any undue advantage in the absence of original stamp paper agreement is missing, flat is not registered, my father who entered into flat purchase agreement with builder is expired etc. 5. What hindrance new buyer can face if we decide to sell off this flat. Your views will be helpful to us. Thank you. Hemant