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How precise do you mean? A frequency with an error of 1% would be easy, an error of 0.01% more difficult, etc. However, since 100MHz is in the FM band your transmitter would be illegal in many jurisdictions.
Audioguru's idea in post #3 is probably the most direct method. You might do well to read this article and use a 20 MHz, 25 MHz, or 33.3333 MHz crystal in the oscillator. Of course all of the tuned circuits would have to be adjusted for the crystal frequency and transmit frequency.