I'm not sure there is enough info here. For one thing, you listed this under ProSeries. Which tax return are you preparing? Year, Form, Whose.

You describe "became a partnership" as if this is a caterpillar pupated into a butterfly. However, didn't you have a SMLLC disregarded Sched C filer, and now they formed a partnership with someone, filed paperwork, submitted for an FEIN? And how did the other partner become a partner: paid something to the other person?

"Assume he had 50,000 distribution in 2020"

Draw or guaranteed payment? From the SMLLC or from the new entity? Was 2019 the final year he operated as SMLLC?

"If he sells the business for 500,000, will he have a gain of 20,000"

Is this as additional hypothetical scenario, or is this part of how that other partner came to exist that created this new entity as Partnership to file 1065 instead of Sched C?

"I am willing to pay for the excel spreadsheet"

Okay, but this is not a Job Board, either. Inside and Outside basis would be easier to determine with the missing details, as a place to start.