You can find 4th remote tractors out there in the salvage yards. I have found parts this way for the ones I have added. Got lucky one time and found a complete NOS in the box out if Indiana back in 2006. Hardest parts to find are:
2 hydraulic tubes from the valve.
4th remote lever for in the cab and tan handle.
Tan flow control, which could be painted. Can still buy the black ones from IH.
And the larger return tube come from the end cap back down to hydraulic pump filters. The rest of the parts can can from anywhere else on a salvage tractor. The bolts or studs to hold the stack on can be made or possibly bought some where, the measurements are in the part book for those.