[Arran]

There are some places that sell materials for guitar amp cabinets and one of those may have a match. What these were covered with was either a cotton or linen, that was then clear coated with a lacquer or varnish, so you could probably find something suitable in a fabric store. Actually replacing the original covering is the difficult part, especially the front panel with the cutouts, that is the part that goes on first.
Regards
Arran