Put your client on extension and have them request a copy of the two returns need to complete the return. Remind them they they need all records leading up to the installment sale to establish basis and all sales escrows.

IRS is not going to have copies going back that far.  What that means, of course, is that your word is as good as theirs.   Back into the numbers with an educated guess, and move on.  They didn't give much thought to the consequences of requesting ancient history, and neither should you.  

Actually you may be surprised at what they have. I order copies of old returns in 2015 and they sent copies back as far as 1964. It was the size and shape of an IBM card. 

If you need more help in getting the numbers here is a photo of my old office ceiling. Often clients would look up when I asked questions like "What did you give to the church last year?" and I decided one day to help them get divine inspiration. 

Taxpayers who need an actual copy of a tax return can get one for the current tax year and as far back as six years. The fee per copy is $50.

https://www.irs.gov/newsroom/what-taxpayers-should-know-about-tax-return-copies-and-transcripts