[Arran]

In answer to your question, yes you could put in a more modern, or vintage P.M dynamic, you may need an output transformer for it though unless it's mounted on the chassis somewhere. Other then the speaker it looks like it's fairly complete, and in decent shape, and one advantage with a console battery set is you have all the room in the world to house a power supply. Here is the schematic from Riders, posted on Nostalgia Air: https://www.nostalgiaair.org/pdfs/Sears-...o-4577.pdf
It's kind of an off one in that it has a 1E7 power output tube, but the two sections are run in parallel rather then push-pull. It's also interesting that it requires 4.5 volts for the tube filaments rather then 2 volts, B+ is only 90 volts, no 135 volt tap. Anyhow there is no rush to do anything with these old sets, put it aside and keep it for a later project or maybe offer it in trade for something more of interest? Bluetoothing or "upcycling" just turns a vintage item into dumpster fodder for later.
Regards
Arran