Have you thought about just trying to seal it up. I saw another member had success with that.
If you want to keep the original look I would try that first because I have not seen any that replicates it i function. The valve is just letting water bypass back to the pump while the thermostat is closed.
If you go with a common brass T you may need a restricter pill or something similar to make it act similar. I think if it was replaced with a tee with a fully open hole, it may reduce coolant flow to the front of the head.
I do have an extra but I’m reluctant to let go of it now