Filing real estate capital gain tax

You can try to use the services of the same person who helped u register the document of sale. U can get a sample sale instance sometime in 1980-1982 time frame.

If the improvements are done post 1981 u can claim the same again, but if they are pre 1981 then the actual cost gets merged in the fair market value. U can claim the improvement of years before say 2000 if the sums are reasonable. obviously u cannot be expected to keep records indefinitely. But some evidence would always help ur case.

The applicable form for Cap gains and bank inetrest is ITR 2

The only evidence of the improvements is if you visit the plot, you would see the improvements. In other words, he inherited the plot, and now one can see boundary and one office-room on the property that he built in 1981. I don't think he has any other evidence.  Is there a government authorized place that can tell us whether the estimate sounds reasonable or not, particularly for the year 1981?  Is he supposed to include maintenance charges too?

Another question: the person had to travel from U.S. to India to deal with the property registration. Can airfare etc. be deducted from the capital gain - falling under the category - expenditure?