1) You don't wanna be tigging that crack. 753 is a Mang Moly tubeset meaning it will become brittle as f*ck in the haz with tig.
Get it fully replaced or it'll crack again.
2) maybe more photographs are needed, but that doesn't look like the crack has progagated through cyclic loading ... it isn't a big sharp sawtooth shaped crack. In this location a fatigue crack would be expected. It looks more like you have some corrosion issues.
Again, replace. Winston!