I have found a circuit diagram:
https://www.service-data.com/section.php/7622/1/a15935. It's a bit hard to follow but seems accurate. I do have a tape but I don't know if there is anything on it. I tried recording on it from the microphone and playing it back, but nothing. Whether it is not recording or just not playing back - or maybe both - I don't know. I was wondering about making a coil with the lead from a jack plug and bringing it close to the head. Would this work? Would it work the other way around to check the recording?
I can't find any replacement heads on ebay unfortunately.