How do I complete 2019 taxes as an executor if I can not find the info? Example-I have 1099-R forms but do not know additional info it asks for (3 yr, general rule.)?

You should review the taxpayers prior-year records to determine the answers. You can also obtain transcripts from the IRS and that might help.  See this general help page from the IRS about getting such information about deceased people's accounts: https://www.irs.gov/businesses/small-businesses-self-employed/deceased-taxpayers-getting-information-from-the-irs

But that is a complex process. Most of the 1099-R questions like that are trying to determine if some portion of the retirement plan withdrawal is not subject to tax (e.g. because it was after-tax contributions, or another exception). If the amounts aren't large you might not bother, just using the box 1 amounts as the taxable amounts. Especially if the heirs/beneficiaries are friendly.

Similarly if you do have prior year records (or obtain them from the IRS) and the 1099-R box 1 amounts are the amounts showing on the 1040, just do the same this year.  That would be the most common thing. Usually the exceptions do not apply.