[Arran]

Ed;
  If the oscillator isn't working I doubt whether the 6SQ7 has much to do with it, I would focus on the 6SA7 and the circuitry around it on the AM broadcast band, particularly the tank circuit for the oscillator. Quite often there is a 47K ohm resistor in that area that can go up in value causing the oscillator not to work below a certain point, if that has already been replaced then I would look at wiring errors, and finally the B.C oscillator coil and the mica caps. The Mica caps don't often go bad but they sometimes do anyhow.
Regards
Arran