Wait, you said it's cracked so you can't play with the cocking lever anymore?
Is it the actual upper that is cracked or just the cocking tube? Most AEGs have the cocking tube separate from the receiver (unlike real steel where it is one piece or welded on). If it's just the cocking tube that is cracked, then you can buy a new cocking tube.
Aside from that, you can try a strong epoxy or JB weld for your reciever. If that fails, get age verified and hope one comes up. I've seen them sold for cheap before in the forums.