But then it's not quite the same amp.

So it all depends on whether your mate just wants a nice working amp or an original looking and sounding Maton. I don't regret having all the changes made to my Sound City head so that it's a very different amp now to the original and sounds so much better (and quieter). Had the OP transformer not required replacing (with a Marshall one for availability reasons) then I'd probably have kept it all original.