Exhaust systems by Do it yourselfers are usually called "Cat Back" systems, meaning the DIYer modifies the exhaust system from the Last Catalytic converter back to the tail pipe so they don't interfere with emission systems on the vehicle.

Is this what has been modified?
From the Cat back?

If so then yes you can get and bolt in stock exhaust, assuming they didn't cut off the end of the Cat

If the Cats are missing then up to you on how best to proceed, gets very expensive to replace Cats

Is it easy..............depends on your skill set and tools, older exhausts tend to have rusted bolts which test your patience, new exhaust pipe can be hard to get into place without lifting the truck up high, i.e. on a lift.