Adjusted Cost Basis Easyguide without the original closing statement

I received the first time home buyer credit in 2008 that has to be repaid.  I sold the house in December of 2020 and am now having to fill in the Adjusted Cost Basis Easyguide, but I do not have the original closing statement and the bank I financed through no longer has it either.  How can I fill out the form without the closing statement and if that is not possible what are my options from here?