What's wrong with the old radiator you can have it cleaned and the inlet and outlet swapped around if that's the problem. I would run a aftermarket cooler for the auto.
Mr hardtako
It’s hard to see but my 12HT return line is Teed into the breather line near the fuel tank Soft line to a hard line along the chassis rail then soft up to the pump.
Terry I take it you have wired the glow screen in to work on ignition on. I fitted the 12ht into my hj75 and never connected the glow screen I’ve had no trouble starting my motor in four years.