[Arran]

Don't use oven cleaner, it might damage the Bakelite, use paint and varnish remover, or if you have a large quantity of it sitting around DOT 3 brake fluid will also strip it off but take longer. The lens is probably held in with tri clips, they are circular shaped with three fingers that snap into holes on the inside of the cabinet, they will pop out by slipping a thin flat bladed screwdriver under them. These cabinets came in both brown Bakelite and ivory painted versions, if the original paint was chipped that may explain the crappy repaint.
Regards
Arran