Ingersoll-Rand 15T compressor

Hello, I have one of the older IR 15T compressor pumps which use to be controlled by unloader valves on the cylinder heads, these have now gone and I would like to convert to start stop like normal workshop units. I guess I will need to fit a none return valve and unloader switch to control the star/delta stater? The only problem is the out let pipe from the pump to the tank is 1" BSP where can I get an none return and unloader valve of this size? Thanks Steve Norfolk UK