These are a thermo-siphon type cooling system. They are a little fussier than a normal system. These do not have water pumps so if water isn't high enough it does not complete the flow path and they boil sometimes.
So, as a start; use a jar of acid flush for cooling systems, rinse thoroughly, drain and refill with 50/50 mix of distilled water and antifreeze.
Maintain about halfway between the top of the tubes in the radiator core and the bottom of the filler neck. That allows room for expansion without coolant loss, top off only with distilled water. Distilled water prevents mineral build up in the radiator core, which cost $$$ to have boiled and rodded out.
Also there is a "seed-screen" (or should be) in front of the radiator. This needs checked, cleaned before starting every time. An occasional blast with cold water through the radiator on a COLD engine, from the bad side is good practice.
If after this is still hisses the radiator may yet need boiled/rodded out.
(yeah, it looks like a water-pump, that's your dynamo)