I want to read and send 40.6835MHz signals with an Arduino. Has someone done it before?
No. That is a really odd frequency.
My first idea was to buy a send receive kit for 433MHz and switch the crystal to a 40.6835 one on the sending unit. Do you think that's possible?
No. The other component values you will have to use ensures that you will need a total redesign. And some expensive test kit to test it.