It's been a number of years since I saw it but if I remember correctly the pin has a large head/plate which comes through a hole in the section and is spot welded in place.
The whole section can be removed if you drill out the spot welds and you could turn a new pin and install it but it's a huge amount of work.
Instead of simply welding a new section on I would probably consider tapping a thread in the original pin and putting a matching threaded section on your replacement piece, if done correctly this will ensure proper alignment as well as giving an additional mechanical fixing to minimise any distortion from welding. Make sure the grade of steel you use is suitable for task as well as welding as many machining friendly grades are unsuitable for welding.
You could turn a guide for drilling and tapping the original pin.
As a side note I would inspect the face where the subframe bushes butt up against, I have on more than one occasion found corrosion around the pin between the sections where the pin is spot welded into the housing., Similar to the corrosion that occurs on the front ARB mounts.