If no taxes were withheld, you could just enter the Form 1099-R as having the amount in boxes 1 and 2a, box 2b Taxable amount not determined marked, code 7 in box 7 and the IRA/SEP/SIMPLE box marked. With no taxes withheld, the e-filing (or a mailed paper filing) will not include the Form 1099-R itself. In this case the taxable amount just needs to be included on Form 1099-R line 4b.
If taxes were withheld, you would need to know the amount withheld to be able to accurately create the Form 1099-R. You might look a the decedent's 2022 tax return to see if it suggests anything that would be helpful in knowing the amount (maybe as a percentage) that was withheld for federal and state taxes.
