Stored in an attic for more than 30 years, the surprise find of the detailed lithograph has been hailed as a 'real blessing' by the team hoping to renovate the vandalised altar, built in the 1880s.
The revamp of the 12m-high, elaborately decorated structure, would become the centrepiece of the 10-year battle to restore the Grade II* monastery designed by Peter's step-brother E W Pugin.
In 1989 the red-brick building fell into disrepair after the Francisan monks decided to leave the monastery, and it was only saved from the scrap heap when the St Francis and Gorton Monastery Trust stepped in to mastermind a £6 million overhaul.
According to the trust, it will cost another £400,000 to return the French limestone altar to its former glory.
Carved by clerk of works Brother Patrick Dalton, the top of the altar was made of white marble from Italy and was supported by eight Californian red marble pillars.
Tony Hurley, the trust's project manager, said: 'Sadly the high altar was heavily vandalised when the church was abandoned, and presently all that remains are the stone steps and the brick base.
'Although the marble and Caen limestone reredos was also damaged, thankfully the majority of it remains intact.
'Although we have plenty of photographs showing how the high altar looked, this drawing gives us the detail we need to ensure the restoration work is accurate.
The lithograph, which is currently being cleaned up at the Conservation Unit of the John Rylands University Library, was donated by Magdalen Donlan, whose grandfather used to be a monk.