. . . I thought a radiator, that is not staying empty till operating temperature is reached was a good enough indicator.
A good enough indicator of what? This statement might indicate a translation problem.
The radiator
shouldn't be empty, regardless of whether the engine's operating temperature has been reached. It should be filled with lower temperature coolant that isn't circulating until necessary. When the thermostat opens, hot coolant displaces the existing coolant and enters the radiator to be cooled by circulation through the radiator. The displaced radiator coolant—that is already at a lower temperature—enters the engine to immediately begin lowering the engine temperature by absorbing heat. To repair systems effectively, it's helpful to learn how they operate.
When a cooling system is recharged with replacement coolant, the system is filled with the thermostat
open, which means coolant fills the radiator, the hoses and the engine water jackets. When the main system is filled and purged of air, the radiator is capped then coolant is added to the reservoir to reach a level between the Max and Min levels marked on the reservoir.
It isn't unusual for instructions to be misunderstood. One impediment in your case is an occasional language barrier. I use three manuals and cross-reference them for instructions when one of them seems unclear to me.
. . . I should replace whole assemblies, because of faulty parts, new injectors because O-rings are leaking, or a new engine, because of a failed main shaft seal?
Injector o-rings, shaft o-rings and seals were designed to be replaced. The thermostat o-ring wasn't.
Those are significant differences that seem to elude your
understanding. Our motos' thermostat o-rings differ because they clasp the perimeter of a thermostat's flange. Somewhere somebody might have replaced that o-ring using a different o-ring, silicone sealant, superglue, some effort, some patience and experimentation because they understood how the system worked, needed to get it done and didn't have the luxury of spare time for theorizing. If their effort creating an OEM o-ring substitute has been successful, I haven't yet found it on the Internet. They might just be out riding instead of writing.
Substitute thermostats have been tried and have been described on this site. Only
your personal effort at experimentation will determine what is possible within your situation.