UPDATE: I no longer need a new crystal as I found the problem elsewhere and have now managed to repair my computer!
Does anyone have a spare crystal for the Apple II they are willing to sell?
According to this thread posting and this thread I need a specific type:
Vishay/Dale XT9UNLANA14M31818 (14.31818 MHz crystal)
Mouser part no. 73-XT49U1431-S (obsolete) (PDF datasheet)
Unlike the postings above I'm NOT converting from an Europlus (PAL Apple II) but have an NTSC version from the start, but it's been unused in storage for 30 years and now there's no colour.