Wow guys, it's only been a few hours and I already have all my questions answered
The schematic does look to be the correct one and the chassis seems to match at first glance. My set is wired with the speaker socket between the two sections of the filter cap, so a choke or resistor should connect to that socket. I went through my box of spare speakers in the attic, and found two with the correct mounting hole pattern and with output transformers attached. I hate to pay the price for a universal replacement so if I'm lucky one of these two will be close to a 4k impedance, otherwise I'll try yet another speaker and look for separate transformer that gets me about 4k total. Now off to measure turns ratios....