As an ex mechanic, I can tell you, a blown gasket does not automatically spell doom. If it happens to be a head gasket, there is a chance the heads are warped. If that is the case, you're looking at roughly an $800 - $1000 repair bill. if it's some other gasket, you're looking at a MUCH cheaper repair. And, if as you say , Jiffy screw left something unhooked, that could have been the cause of the damage to said engine, and if so, THEY have to pay to fix it.

the Muffler, hit Mineki (or however ya spel it) A new muffler, installed, will run you around 60 bucks, installed.