What would be a good sub for a .1 1kv white tubular porcelain cap would a ceramic disc be just as good ?

No. Ceramic caps drift with temperature and change value with applied voltage. Use a plastic film cap.

Mouser and digikey sell 1.6kV Panasonic film caps that are pretty good. If you can get the capacitance you need from one of those that'd be what I'd do.