I have found this image and wonder how to calculate or determine "Cm", because I like to simulate this circuit in LTSpice. Also should I use a Mosfet or a voltage controlled switch?

Can someone help me?

 

This might help you get started:

It looks like CM detunes the LC resonant circuit (or tunes is, can work either way). The value of CM would depend on the values of the inductor and the other two capacitors, the desired depth of modulation and the Q of the circuit when the switch is open and the Q of the circuit when the switch is closed. I will not be easy.