[Arran]

I would suggest washing the chassis first so you can see how much rust there is. It looks to me like a rodent got into the back and used the chassis as a toilet, believe it or not I've had success using paper towel soaked in vinegar to get the  heavy rust off, I tried it on a Stewart Warner farm radio chassis and it got it down to bare steel, then I treated the areas with phosphoric acid based rust remover to stop them from rusting again, if I can get it clean enough I may not have to paint the chassis. Depending on how motivated you are it may make sense to strip the chassis down, if the power transformer is still good. Any pictures of the cabinet? 
Regards
Arran