For the 72 MHz band, here is a picture showing the frequency and the "channel" that goes with it.

You can remove the channel 15 crystal from the receivers you linked and replace them with the appropriate channel 51 crystal.

The channel 51 dual conversion crystal you need for the Hitec Electron receiver is http://www.hobbypeople.net/gallery/759951.asp
There are lots of places to buy them. The Hitec manufacturer part number is 6677251

Hitec dual conversion receiver crystals have a blue tag. Their single conversion crystals have a green tag and they will not work in the Electron receiver.