Failing oil supply to that bank of cylinders and badly worn cam lobes, possibly? Does anyone know, if the M113 is run low on oil for a long time, which (if either) bank of cylinders wears first and worse?
The seller obviously believes/hopes to persuade possible buyers that the low revs are caused by a tappet problem, and he's changed one, so presumably they're quite noisy. I can't help wondering how a tappet problem would restrict the revs unless it was VERY severe, though. Unless the engine when running sounds like several skeletons break-dancing on a corrugated iron roof, on general principles it sounds more likely to be a fuelling problem to me.
If it's not the tappets, what else might it be?