Flat area not mentioned in the agreement

Dear Sir/Ma'am, Good day. We are planning to buy a flat in mumbai, but in the Agreement bought in 1991 by owners currently residing, the area of house is not mentioned in any form (neither carpet area not built-up area). The area is not mentioned in the "INDEX 2"either. However, the owner has paid stamp duty for which the receipt is present in 1991 & 2009 of the flat based on the area mentioned in the society letter. Also, we have a society letter mentioning the super built-up area of the flat, plus the maintenance bill of the flat which mentions the super-builtup area. We need to buy flat now, but we want area HAS TO BE mentioned in the original "Agreement". Is it ok to do deal and make new agreement which will have the area of the flat in it with the help of society letter.This would not create a problem while considering the same flat for redevlopement. What can be done in this case? Is it sufficient for the area to be mentioned in the Society letter and manitenance bill? Can I do anything more to solve this issue? Thank you and awaiting your response.