Just an update on how this turned out: I replaced the broken thermostat and tried it out with the old radiator. It did not really seem to make a difference (maybe a little but not definitive). I then put in the gemini radiator and it now seems all good.

I was surprised the car was only running a bit hot (2/3) with a physically broken thermostat that was decidedly stuck closed, and that replacing the part did not seem to make a difference.

gemini radiator wasnt that hard was it, now time to try the n13 pulsar/s13 set up... things get interesting there

good to hear ytou fixed it

Cooling systems are one of those things that we seem to take for granted & we neglect them at our peril.

When I got my 1200 coupe it was clearly in trouble in the cooling department & if it wasn't for a strong headwind & constant light rain I would have suffered quite some grief on my trip home with it.

I brought all my unknowns under control by replacing all the rubber hoses, including the heater hoses as well as the water pump [loose bearing] the fan belt [it was inexpensive] & the thermostat.[I didn't like the look of it.]

The radiator core was clogged & was replaced. I removed the block drain plug & got stuck into it with a long thin screwdriver to dig out as much crud from the block as possible, flushing it all the while.
The heater got a reverse flush too & it works a treat.

It was finished off with a cheesecloth condom type of coolant filter in the top hose fitting & the only problem I suffered since was about 3 weeks back in Victoria while driving in that "devastating heatwave" with 44C temps [112f] A faulty thermostat raised the coolant temp to the 2/3rd mark but this has since been rectified for the sum of $16 for a replacement.
It has otherwise always run with the needle right in the middle of the guage at all speeds & in all temperatures & still does now.

Take care of your cooling system & it will take care of you. Neglect it & you could be up for a replacement engine as well as the cooling system repairs that should have been done earlier.