Do you want to be able to change the frequency between 20 and 40 Mc or do you need a fixed frequency? For variable frequency you'll need to make the suggested VCO with PLL. An option would be to make it run at 10 .. 20 MHz and then build a simple frequency doubler. At the lower frequency range, making an oscillator is much easier and with the reduced base frequency it you have a larger selection of available PLL's.