Heater Valve, DOHC 124- Original #4472892 is NLA- We've been using the X1/9 heater valve, #4208998- This valve is usable, but it is necessary to kink the control cable/modify the firewall for proper fit. The latest development is the Beck-Arnley part, #130 136-1013. I'm not sure what this originally fit, but it can be installed in a 124 Spider with only minimal enlargement of the opening in the firewall. As long as they are available, the Beck-Arnley part is the way to go. BTW- Heater valve seals do not come with the Beck-Arnley valve- Get those through Fiat, order two P/N#4176872.
Heater Core, SOHC 128- In a pinch, 128's can use the Yugo heater core & valve, any year Yugo will do. Some modifications, depending on models used, may apply, but this is a very viable alternative due to the availability of late-model Yugos in the boneyards.