Ares Tar 21 help

[Sealcon]

I'm looking for some help regarding my brothers tar 21 if anyone could help me out?

He bought it pre owned and the bolt catch has been stuck towards the rear of the rifle since he's had it. The piston stripped the other day so I took it apart to fix it but I've no idea how to reassemble the bolt that goes on top of the gearbox.

The main issue is I don't know what to connect that spring at the top of the gearbox to, any help would be greatly appreciated.

Thank you.

(Parts are pictured)