First, I would like positive confirmation that this capacitor actually failed. In 30 years of dealing with them, both in M49 and U47/48 I have never had a single one fail.
But, if indeed the cap is shorted (?) your choice will audibly affect the sound of the mic: some swear by wet tantalums, others by oil-soaked contraptions. A very good film and foil (not metalized poly-prop!) will add a bit more clarity without thinning out the sound, while those film & foils made with other metal films typically sound more pointy in the mids and highs.
Contact me privately if you want to replace with original material-these days collectors collect everything...