Once the loan is forgiven, in order to write it off you would need to report the income. Since it is tax exempt, that is a book entry: Debit PPP Loan Payable, Credit Tax exempt income.

If the loan is not forgiven until 2021, then it is still on the balance sheet as PPP Loan Payable as of 12/31/2020. There is no income yet, so there is no book income nor any M-1 adjustment. That occurs in 2021.

The expenses paid from it should just be expense deducted when paid. 2020 mostly.