You probably damaged the seal, use Hose clamps in that size to bind it on the piston after you installed it carefully. With carefully I mean that you don't stretch it that much and work it carefully with a pick on the piston.
Then this Field Tech is dumb, sorry. The computer compression test never fails and is not even checking the compression. You have to do a manual compression test through the glow plugs or even better through the injector opening.
Are these field techs from Bobcat of North Texas?
Which harness was replaced? Because there are 2, one from the ECU to the ECU Harness and then the Ecu/Engine Harness.
If you look on the PDF I attached, it is either Part #11 or #41. Which harness was replaced?