I'm rebuilding a GA case 2.0 L motor that a PO had put hydraulic cam and lifters in. I'm removing the hydraulics and cam and putting in the Raby 9590 cam kit with solid lifters. While prepping the case I pulled out the oil pressure control valve (the second valve on the flywheel end) expecting a spring and piston but found what appears to be a copper plunger with no travel (essentially a plug).

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Based on reading recent threads I suspect it is ok to leave it this way, but was wondering if that is any advantage to reverting to the original spring and piston?

There is no way I would leave that stuff in my engine. Go back to the spring and plunger so that it can do it's job. It is there to regulate the pressure.