Kohler CH20 pulls down under load

I just bought a JD F680 w/54" deck and will be picking it up in a few days. Owner reports mower has a new short block, PTO clutch and carb installed by dealer w/ 2 hrs on them and it runs/drives fine but revs pull down when the clutch is engaged and you start to mow. The clutch and carb were installed separately by the dealer as attempts to cure the issue and eventually the previous owner and the dealer grew pretty aggravated with each other and threw up their hands and gave up. JD owner became a Dixie Chopper owner and I became a JD owner.

I am thinking governor but can't imagine that the dealer didn't try adjusting it 1st thing but the old owner doesn't think so. I would also guess carb but that has hopefully been addressed. Since it was a short block, it am wondering if the valves were reinstalled a little tight and maybe it's down on compression? I don't know yet if this engine has the smart spark feature but am wondering if this could be an issue since I read about a similar problem in an old forum?

I'd like to hit the ground running as soon as I get the mower home and have a list of all possibilities to check. I really don't expect this to be a major issue since the heads were supposedly checked out and OK, the engine has a new factory short block and carb and the work was done by a (questionable???) dealer but do need some help on possible ideas that would cause this symptom...any ideas out there?