Some of the field coil speakers had metal or phenolic spiders that were screw mounted and allowed for voice coil (re)centering.

If you end up having to replace the driver, consider checking Ebay for vintage extended range 12-inch drivers (some with field coils) from open back consoles. Be sure to match the VC impedance (nominal 8 ohm would work fine). If you go with another field coil driver, be sure to end up with a similar inductance and dcr because, as you noted, the field coil is also the power supply choke. If you buy new, you will need to find an extended range driver with a Qts in the 1.0 to 1.4 range and an Fs in the 50 to 60 Hz range. Relatively high efficiency (mid 90s) would also be desirable.

Out of curiosity, what output tube(s) are used and is it PP or SE?

Nice project.

Regards,

AstroSonic