First off, tha capacitor is part of the electrocoupler firing circuit and is not needed for anything else.
Step 1 is simply cut the leads of the original capacitor to make it open circuit.
Test the board, If the board works fine without the capacitor and since many people never use o even read the manual to go through the sequence of firing the couplers, you could just leave that part out. In fact, on some boards used in units without electrocouplers, there is a line on that circuit board and they just cut that entire section off.
You can replace the capacitor and then attempt to fire an electrocoupler if you truly want to fix it for full function. But I would test first to see if the diode had failed.
From the looks of it, this may have been a built in or dented defect in the capacitor and just burned through the side of the can. That said, again, in theory, that condition of the capacitor in full short internally, put huge short circuit current through the charging circuit of the capacitor and may have killed it. If you feed the new cap with AC instead of DC (because of a failed diode) that could get ugly.
