That's how they come. The street model NSR50 also has a smaller radiator than the NSRmini racing model, no thermostat and no fans. The temps flutuate between 70-80.
The bike doesn't come with a temp guage just an idiot light so I got a temp meter straight away. The NSR250 radiator looks like it would fit but has twice the frontal area of the NSR50 unit. Without a thermostat that could be a problem. I also don't want to buy the HRC NSR50 radiator as it does not fit my long term plans. I guess I will try flushing the system and running water wetter instead of antifreeze and clean the radiator fins.