Keep in mind that for any difference in capacitance, you can compensate by tapping to primary differently. I believe if you have a higher capacitance than is needed, you'l want to shorten the primary, and vice versa. I agree with WTP Pepper that two 100nF caps will work in parallel, but you must be careful about the types. They must be rated for more than twice the transformer voltage, and must be designed for high frequency. There are several types that are very lossy at higher frequencies, and these will prevent the tank circuit from oscillating properly.
Regards,
Der Strom