TOPIC:

I might be wrong, but theory says the crystal is only setting an oscillator frequency for the carrier wave & it shouldn't matter a toss whether the final transmission is amplitude modulated or frequency modulated - try some '40 FM' crystals & let us know what happens (I don't have any 40MHz gear to play with at the mo)

here is a tread I found about am/fm that makes some sence

fixed the link

The cristals appear to be generaly the same but thats only usable for the receiver asfar as I understand for the transmitter it works differently and then there is the problem that several manufaturers use there own modulation and you only can use their cristals.

Cant open the link, but the animation says it all - 'AM' is amplitude modulation, ie the signal is changes by size of the 'bumps' of the wave, 'FM' is frequency modulation ie signal changes by how many 'bumps per second' there are.
The AM or FM bit is how the signal is modified & transmitted, but first there is an oscillator that generates the actual base wave to start with, the frequency of which is 'set' by the crystal (There's more components involved, but the crystal just makes it more accurate).

Of course, this is all blurrb, best thing to do is try it for real to see if it works