The plumbing aspect of fixing it is probably pretty minor, but it may take a bit of designer time to fully analyze the solutions that would make it work without interfering with the operation of the other radiators and getting decent room-to-room temperature balance. The system topology looks as if it was a 2-pipe steam system (see the picture below) converted to pumped hot water, which is successfully done every day by competent hydronic designers.
Looking at your simple sketch diagram, installing a valve (or even a narrower section of pipe with a ball-valve for throttling it) at the U between the supply and return sections of the the main loop puts an impedance to flow on the main loop to impose a pressure difference to induce flow through the radiators. If each radiator has a valve (as in your photograph) the flow balance from radiator to radiator can (and must) be tweaked. A really fancy/nice system would use adustable thermostatic valves for each radiator which would be easier for balancing the system, with less interaction between radiators during adjustments.
On the existing system as-is I expect you'll find all of the gate valves (like the one in your photo) for all of the radiators in the fully open position, which is lowering the pressure difference to something really tiny at the two that have no flow. Closing down the valves on the other radiators slightly may raise the flow through the cool radiators sufficiently to work at least a little bit, but it can take a lot of tweaking to get the room-to-room temperature balance perfect. Barring thermostatic valves, ball valves would be far superior to gate valves for tweaking flows. Gate valves will become eroded to where they can't fully close when used for flow adjustment, and it's possible that the system was once tweaked into reasonable balance 40 years ago, but that the flows have changed over time as the gates eroded away. (Just a WAG.)