The Hoops have had two bids for the Scotland international turned down by the Easter Road side, but are determined not to pay over the odds for McGinn, who is out of contract next summer.
But it has emerged Celtic could look to sweeten the deal by offering a player-plus-cash arrangement.
Johnston is understood to be a possible option, but the 19-year-old is also wanted by St Mirren and Aberdeen.
Talks have taken place between the Hoops and the Buddies, with Scotland Under-21 star Johnston said to be open to a loan move.
Neil Lennon’s side are in the market for a wideman following Brandon Barker’s return to parent club Manchester City.
There had been speculation that Scott Allan, who spent a successful period on loan with Hibs during the second half of last season, might form part of any final agreement but Lennon insisted last week that the midfielder’s name had not been mentioned.