[KCMike]

 Hi murf, I'm no expert by far, but if you have any 250mmfs I would remove one leg of the one you plan to replace and just jumper a new one in to see if it makes a difference. The voltages that would be minimum for that part of circuit I can't tell you for sure.
Heres a place that offer high voltage rated micas.
http://www.justradios.com/mica.html
Good luck.