Picked up this spacer on the weekend from MX5plus in Brisbane, Australia. This allows the thermostat to be moved from the front of the engine to the rear to allow for a more complete coolant distribution. The original configuration meant the cylinder closest to the firewall flowed out only through the heater core which is insufficient to keep temperatures even across the head.
You will also need a Kia thermostat cover with the part number: 0K24715172A
Thread detailing a similar installation from miataturbo.net
And another from Australia.