depending on what the crack looks like perhaps some small pieces of metal
"foil" type material could be wedged into the crack to at least slow the
escape of gasses.  while this is not a fix it might buy you some time.  i
would think something soft like brass or alum might do the trick.  if you
get a loose fit when cold depending on the anatomy of the situation, as
things heat up and "swell" you might get a tighter fit.

jonathan