This morning, I ordered a new crystal for the receiver. The specification is per the Tait T11, and with a frequency of 31.265 MHz, this should provide a clean receiver frequency of 51.830 MHz.
The crystal (including one spare) and postage came to €100, which I thought was a very reasonable price.
This brings us another step closer to the end product. Our next step is to source a new antenna for the repeater, something like a Diamond CP-62.