Weld it again and put in a brace plate or two at right angles to direction of the crack
In theory there should not be any movement between the engine & the frame , if the engine is bolted solid to the frame
So check the engine mount bolts are tight
I can not visualize the exact set up from the IPL but consider fitting thackery washers to one of the mountings so it can move without flexing the exhaust
PS Thackery washers are like a double split washer usually found between the rockers on car engines to allow a small amount of movement along the shaft .