I had a hole in my J pipe when I got my deuce. I had a laundry list of repaiirs to do and money was tight so I made a temporary repair and it is still holding. I do plan to repace the pipe this spring.
I took a section of a 3" exhaust tailpipe that had a 90 degree bend. I cut the section out of the pipe and then split it in half with a sawzall
Now I had a cap for the outside of a 3" elbow.
The problem is the deuce has a 3-1/2" pipe that is the elbow.
I took an old serpentine belt idler which was 3-1/2" (the flat one) and clamped it in a vice and using that as a mandrel, and employing a mallet, "reshaped" the 3" elbow to 3-1/2".
It fit pretty well. I took 4 hose clamps and clamped it in place. I sealed the patch with silicone seal, which believe it or not, it is still there and has not burned out.
Worked for me but it IS A COB JOB.
As I said I am pulling the pipe this spring and if it is not really solid, I will replace it.'
The cause is not capping the stack and letting water sit in the elbow so if you replace the pipe and do not cap your stack you are simply repeating the process of rotting a hole in the pipe.
Just my two cents
RL