if you are interested in having a completely custom steel body made, message me. I have a contact who would likely be willing to do it (but it would cost quite a pretty penny as it would be custom machined).

Check your recoil buffer (a rubber part inside your gun that softens the recoil impact of the bolt).
A friend of mine had the recoil buffer pad of his mp7 beaten up from use, then the pad get loose and falls out of the gun. He shot a few rounds without the buffer pad and cracked his receiver.
If your pad is lost or damage, it could cause the crack or worsen the crack.