[Chas]

Getting the core reinstalled and centered properly without the jigs likely originally used would be a challenge! and beyond my abilities I am afraid.

Can be done without a jig using flexible shims to align the voice coil. Yes, some "machine shop" equipment is required to work with a pressed/welded electrodynamic speaker.

The field coil is always a better choice than a "filter" resistor.

IMHO not to worry about the apparent mismatch of the resistance of the "new" voice coil vs the old one...

From what I see, (I should reference the schematic) is the original speaker has a hum bucking coil, if the replacement has that coil fine. If it does not. there may be a faint residual hum that can be dealt with using a 10 to 25% large cap after the "2nd" cap in the filter array. Also try reversing the current flow in the filed coil regardless if there is faint hum.

There is tolerance in values used in an everyday radio that will allow for variations in component values to some degree. Lash it up and try it before committing to a permanent installation...  chas