I've done power upgrades on more than a few 3126's. To go from the 210 hp engine, it's a simple injectors/turbo/reflash. That'll get you 30 hp and about 200 more in torque. Beyond that, it's internals. To get to the 300/330 range it's pistons/damper/clutch & fan/radiator, etc., plus the above. To go beyond that, you're into the marine componentry. And you'd be on your own to get an even bigger radiator and oil cooler in there. Beyond 330 horse, lifespan drops incredibly fast.
The 3126 is not a heavy-duty engine by any stretch. At stock power ratings you'll get good life out of them, but they're just not robust enough to push hard. You can get them to the 400 horse mark, but I wouldn't want the tradeoff of reduced lifespan.
Which engine are you attempting to uprate?
P.S. CAT parts are outrageously expensive. The 210 to 250 uprates cost me about 3 grand having my techs do them. I priced the 300/330- with internal labor it would be over 10 grand.