There are some Pricey Engine Re-Build components such as expensive H-Beam Con-Rods... but because the GM Atlas 4.2L sports a 24 Valve DOHC Engine Head and Valve Train using the VVT (Variable Valve Timing) feature between 0-25 Degree on the Exhaust Bank... there are NO After-Market Cams or Forged Crankshafts that are available as "Drop-In Parts" like those available for most LS V8 Motors. Your best bet is to view the information via THIS Link... and afterwards, enjoy driving your relatively Stock I-6 Powered SUV after having @limequat run a $99.00 "Tune" on it for you:
Just sent my info and payment to Lime-Swap for a pcm enhancement! (loaner swap) woohooo!,... fingers crossed Heres a list of my request,.. am I forgetting anything ? 2005 Trailblazer 4.2 with 140k miles Gear ratio is GU6 or 3.72 Tire size is 265/75/R16 Tunes: Shift Management (shift delay)...
...there ARE only a Handful of YouTube Hot-Rodders that have Monkeyed the Horse Power Out of this Straight Six Motor to the "Tune" of nearly 500 Horse Power... but they have employed Hard to Build Turbo-Charging Rigs in Non-GMT360 Vehicles (Checker Cabs and Studebaker Larks) and doing THAT only after investing a Boat Load of $$$ and a HUGE Trial and Error Effort in order to accomplish this Daunting Feat.
"Subdue Your Appetites... and You've Conquered Human Nature..."
From Charles Dickens'... "Nicholas Nickleby"
A search throughout the site will yield quite a bit of info regarding turbos, injectors and Youtubers that have done a few things with this engine. But as @MRRSM has said, there are practically no off-the-shelf performance parts for it, not even head bolts or studs.