This is the problem with Vetus goosenecks, waterlifts and sea strainers. You will end up buying
a new one an reinforcing it before installing. If the cracks/splits on the seams originate where a fitting is screwed in, put a hoseclamp around so it can't split. You can also use some sanding
to rough it up plus JB Weld to reinforce it.
For temporary repairs
: try a bead of 5200 over the split followed by something to hold it in place (push it onto the split) like hoseclamps, tape etc.
I don't know another manufacturer for the goosenecks so you're probably stuck with those or you would have to redesign that part of the exhaust
I also have that "tailpipe repair tape" for cars aboard. It needs heat to cure so this won't be automatic like with a car but a heatgun works too.