"Gor" is blasphemous and "Cor" isn't - I'd say they are completely equivalent to each other.

"Play" fits the melodramatic quality of the song much better. I think thta;s what Dave Bryant sings but of course "...play, Gor Blimey" and "...plague, Gor Blimey" sound indistinguishable.

"Better than a play" is a very common expression to describe some notable event in real life. "Better than the plague" is not an expression which is heard very often.