Cat C7 starts, idles, but won't rev up in gear.

Diesels require almost no fuel to idle. Clogged fuel lines and fuel filters would be my first suspicion. Once you power up a diesel will become starved for fuel and have no power. Of course old diesel tractors and swathers don't have all those flashing lights and codes. We see that a lot on farm equipment. Change the fuel filter as posted before. Then check fuel line for any clogs.