While in there, replace the o-rings that are on the front water pipe as well.
On the hole - if you can't find another set, maybe you could do one of the following:
- "wrap" that section with some galvanized sheet metal and on both sides use some exhaust header wrap to act as a gasket, then hose clamp it on. Probably could use header wrap around the entire section, covering the hole too, then clamp the metal wrap over that. Or, put a small galv patch piece loosely over hole, then wrap header wrap around the whole area, then secure the ends of the wrap and put a couple hose clamps directly over the repaired area.
- an exhaust pipe place could probably match the size dia pretty close and have them weld a short section in to repair
- cut that section out and cap it off. Not sure what that would do for performance but others on here will hopefully know how detrimental this may be.