//Does the agreement for sale have to be stamped properly (in mumbai) before it can be admitted as evidence? //
Insufficiently stamped document can be admitted in evidence, by collecting required stamp value with penalty, as it is rectifiable defect.
You must understand the difference between filing the document and admitting the document in evidence. Admitting the doc in evidence arise only during trial.
//Agreement executed over 7 yrs ago. And still not stmaped properly. what are the implications if not stamped?//
If not stamped, court can impound it for collection of deficit stamp duty under the provisions of the stamp act.