I have a crack in the frame side where the cross member that holds the transmission mounts to the frame.

I assume the frame is sections tig welded together at the factory.

I assume the sections are cast not extruded .m

Is there another option other than cutting the frame apart and tig welded a new section in ?

This is an important mount as it shares the weight of and keeps the transmission inline with the drivetrain.

How can I tell if their are other defect in this section.

 

That is a strange crack. Must be have been there from the fabrication. Or is there an event?

I wouldn’t feel so worried, it has 2 points of attachment on both sides. I think it caries mostly the weight of the trans.

if you want it repaired TIG welding is an option, but as it is aluminium you have to find a aluminium welder.

Or if that is to complex (I don’t think so), perhaps a metal plate can be fabricated between the frame and the bracket.

 

If that is a solid cast bolt boss, which it does appear to be. Did you remove the bolt to see how far past the crack the bolt extends? I have drilled and tapped the hole deeper in a solid cast boss, and then installed a longer bolt in this type of a situation before on equipment that took a solid hit, which in turn transferred the energy to the bolt and cracked the casting.
I have seen these castings be hollow past a certain point, you would need to investigate that further.