inboard rpm problem

I'm looking at an E38 that is quite nice in many aspects. It has the original rigging and original main, which, not being an antique collector :p I'll probably replace. But the thing that's giving me pause is the engine. Several of the classic repairs have been performed; water pump, bigger...

My Universal inboard diesel has recently weakened for unknown reasons. At full throttle it reaches 2000 rpm but the boat is barely moving. It used to max out at 3000 rpm. I'm new at engine repair. Any recommendations?