maximum voltage that a colpit oscillator can withstand
The maximum voltage you can apply as Vcc is going to restricted by the limits of the components in the circuits. 

I.e. oscillators that use op amps will be restricted by the op amp's maximum supply voltage and input voltage, go too high and you'll fry the op amp. You can get op amps which can handle large voltages, you'll just have to search for them on a distributors website (like element14 or rscomponents) and they may cost more than your regular op amps.

Same goes for any other active component like mosfets and BJTs, though they may also be subject to current limits too.

However, for a high power application, you can only increase the voltage so far, and you're unlikely to find op amps that can handle 1000KV. What you need to do is also increase the current flowing into your antenna, the same way you need to increase the current going into speakers to increase the power.

A better solution is generate your oscillations and then pass them into a current or "power" amplifier to drive your antenna or load.