[Cheese Man]

The trigger box is not attached to the barrel...

That looks like a locking pin in the stock trigger mech - the cheapest way would be to put it back, if you can't find where to put it, just buy a new mech/box.

 

[Cheese Man]

Unfortunately, no. My first upgrade, with barrel and hopup, was the Zero Trigger, and my stock one never fell apart or wore out, so I didn't take it apart.

 

[Cheese Man]

Then just use the trigger mech...