"In box 2, they reported the entire distribution amount from box 1 as taxable, but I usually don't see a G code if going into a Roth, I usually see a 2 for under 59 1/2. I called the 2020 401k broker, but they said that's how they report it. Even though the replacement check wasn't distributed until 2021, they won't amend."

Let's understand that a lot goes wrong with these events. The employer gets it wrong, the receiving entity gets it wrong, the taxpayer gives the wrong instructions, the broker/agent acts wrongly, etc. Yesterday's Dave Ramsey has a caller who asked the broker to convert "a few shares" from IRA to Roth, and instead, the entire account got converted.

I read in here that a Check was lost and reissued. That means the 2020 1099-R would include withholding, if payable to the taxpayer. But Code G indicates sent directly to the other brokerage. And it assumes a Qualified Plan. That means the issuer was not notified this was going to a Roth, and they are not always aware of what is happening at the other end, anyway. It would be reported as not taxable if it first went into a traditional IRA. It seems it went to Neither Trad or Roth timely? This is Important:

"so the client didn't open her Roth until March of 2021 when she received the check."

She Received is not Direct Transfer.

The issue of Timeliness has not been answered.

Get the Form 5498 for all accounts that were part of this, to see what the receiving entity considered happened.

Let's examine what would happened across a year end, as long as it meets the timeliness for Rollover:

She has a 2020 event as Distribution from the 401(k). Within the required timeframe, but now in 2021, she presents the check to the brokerage to establish a Roth, as a Rollover, which also makes it taxable as a Conversion (not like-account to like-account). That is reportable and taxed in 2020.

There is no G event if she had access to the money as a check.

"It's on the 2020 return, but just as a direct rollover, non-taxable event."

Not direct, and Not non-taxable. It appears 2020 needs to be amended.

"can I create a substitute 1099-R using the Roth IRA's fed ID code"

That ID has no applicability here. You are not issuing the 1099-R. There already is one, for 2020. If you confirm it went into a Trad IRA first, then there will be a 2021 1099-R from the brokerage for the conversion in 2021 and the 2020 1099-R would be correct as a nontaxable event.

"or do I have to fill out 4852 and attach?"

No. Deal with the brokerage.