Cracked Magnum Heads - Junk or Not?

If you put hard seats in those, int and exhaust.... Then there's probably nothing to worry about.

But if you only install hard exhaust seats and that crack is towards to int seat enough...it will come back into the int seat and cause issue.
Ablot of x,j,u,o, or whatever heads have a lil crack on the plug boss, never an issue though.

Heads can be repaired and be ran longer than the owners lifespan in a lot of cases.