Marvin Chassman is one of few fans to have heard Procol Harum play Repent Walpurgis with stand-in organist, Don Snow (May 1992, New York's Academy of Music):
At the end of the show Gary said that they only had time for one more song before the hall closed, and put it to a vote between Repent Walpurgis and AWSoP, as measured by clapping and noise. Some late stamping put Repent over the edge
I don't remember anything particularly exceptional or notable one way or the other about their performance of that song that night. Maybe they really didn't play it any differently than usual. Don Snow was OK as far as I remember though I never saw Matthew Fisher play this song, or any other. The only time (or times) that I saw PH play this song before, Chris Copping was on organ (I also saw them on the tenth anniversary of AWSoP with Pete Solley but I don't remember that they played Repent Walpurgis). Don Snow was every bit as good as Chris Copping. But in any case the guitar playing is about as critical to that song as is the organ playing.
Note that this was a few weeks before – and with the same line-up as – the 1992 concert with the Edmonton Symphony Orchestra, which was televised in some countries (not the US as far as I know) and is available as a bootleg video. Watching the tape, one can see that Don Snow is playing from sheet music ... so I assume he was doing that when I saw them.
More Walpurgis revels here