Yes it does sound like the governor. But first are you sure that the throttle isn't creeping closed? I would first squirt WD40 or equivalent in all exposed parts to loosen up anything that might be binding.

But if that doesn't work, your options are limited in Horta. I suspect a governor/injection pump rebuild may be your only solution. It will be expensive and you need a good diesel shop who specializes in these to do it.

David