Compare the hydraulic system of the 4118https://manuals.casecoltingersoll.com/PartsManuals/Ingersoll%203118,%204116,%204118,%204120%20Parts%2 0Manual%208-3200_watermarked.pdf
to the hydraulic system of a 4018PShttps://manuals.casecoltingersoll.com/PartsManuals/Ingersoll%203016PS,3018PS,%203020PS,%204016,%20401 6PS,%204018PS,%204020PS%20Parts%20Manual%208-3121_watermarked.pdf
and you will see that everything in the hydraulic systems will need to be replaced to include the travel control valve, the rear PTO, the reservoir and the pump, big expensive job!
The only favorable thing is that the P218 engine in your 4118 has the same length crankshaft as a P218 with an electric clutch which you will need along with a PTO switch and the correct wiring harness!
Think I got it figured. This tractor has had several things cobbled together incorrectly. I think the rubber engine mounts were actually installed incorrectly, and I replaced them the same incorrect...