flat buyer purchase agreement. Please guide me.

Flat Buyer purchase Agreement- I had purchased shop from builder in 1976 in New Delhi. He handed over possession to me with possession letter dated 1982 stating that the possession was given to me in dec 1978 & also stated that I have paid him full consideration for it. I do not have flat buyer purchase agreement. Now I think my matter will come under sec 53A of T.P. act? I think sale deed cannot be made as it is DDA lease hold property. Now I want to approach the builder and ask him to sign the flat buyer purchase agreement which will have to be registered.From which period time will began to run(period of limitation) againt me for signing the purchase agreement as I want to ask the builder to sign the purchase agreement now.Kindly inform under which article of limitation act it will come.