It looks like a fatigue crack, you can see where it has cracked at the weld with a little rusty spot. The corrosion doesn't appear to have thinned the metal significantly.
Being brutal it is a fairly ordinary industrially-produced frame made of good quality materials. Probably I would buy a near replacement and transplant the bits.
Being brutal it is a fairly ordinary industrially-produced frame made of good quality materials. Probably I would buy a near replacement and transplant the bits.