Sounds like an injector. BUT. A knackered injector can knacker the piston rings if it is ignored too long.
Will the owner let you do a few checks?
Check 1: get it running and then loosen off the fuel pipe union at the injectors one by one then , if the engine slows down, retighten that one's OK continue till you find the duff one ie. it doesn't change the engine speed.
Check two: Remove the duff one and get it tested or replace it with one from the scrappy. As the engine doesn't smoke, we would hope it runs OK with the new/second hand Injector. BUT it might start chucking out smoke and that means the rings are shot from running on a non firing injector.
Happened to me on No.1 cyl on the newer Someca 780. Luckily it is possible to change the rings on No.1 cyl without splitting the whole tractor. Phew!!! Cylinders 23&4...Assume position....

!!!
If you need something done quickly just ask a busy woman then get the f"k out of her way.