While your guy is doing compression test, I suggest do fuel injector test BEFORE final decision to do deep, expensive repairs in it. A weak injector or two will give identical running problems, as will fuel starvation from clogged filter! Since rigs like yours sometimes sit for months, condensation, waxy sediment, ect happens a lot, none of which makes it thru filters and inlet screens.
Info on what the tech does for comp test: