[siggypoo]

As the title states, I received a Javelin M4 in a trade. It is front wired with a PEQ box, but with no battery or charger I don't know if it works. Before I get to that point though, there are some structural issues I'm hoping to have help with.
First and foremost, the upper receiver slides into the lower receiver and is held in place by the front pin. The pin slides out at a moment's notice. In the receiver there's a recess for a spring and retention pin, but it's empty. Also the receiver pin itself is smooth and has no slot or indent for a pin to lock into, so I'm wondering if the pin is wrong for this gun.
With the receiver mounted, the rear of the upper moves rather a lot. There's definitely space between the upper and lower lip that causes the upper to wobble up and down, left and right.
The cocking handle just rests in the upper receiver and is not held in place by a spring, and there is no inside ejection port cover for the cocking handle to attach to, so the piston and hopup area is exposed.
The barrel rotates a few degrees freely, and I presume (hope) tightening the barrel nut once I have a tool will fix that.
Finally (for the moment), the shoulder stock doesn't extend. And has no latch. Peering inside the latch hole it appears the stud that locks the stock into position has broken off and is jammed at an angle that prevents the stock from retracting or extending. I assume I can Dremel the protruding material out to remove the stock and replace it. The buffer tube seems fine.
So I'm hoping to find out from you guys where I can get replacement (or compatible) parts for this gun, what battery and charger I should look for, and what you might suggest for the issues I've mentioned. Especially the upper/lower receiver wobble.