If you want to report the income as received in 2019 instead of 2018, go ahead and include in your 2018 revenue, then create a "Miscellaneous Expense" for Income included on 1099, not received till 2019 for the amount of the payment. The two will net to zero and you can then claim the income on your 2019 tax return. Or you can just include it on 2018 and have a discrepancy.
