The motor mount bolts should only be tightened to their torque spec, never above. Your cases are cast aluminum, so the metal bolts are much stronger. Clean and preheat the area to help purge the impurities before welding. And clean the area with stainless brushes to remove the oxidation, even if TIG welding. The outcome will be much better. I'd also tack it in place, then install the bolt, gently tight, then finish welding it to avoid distortion.