Should I try and re-attach the old mushroom valve for the time-being?

 

Sounds like there will be nothing to keep it in place as either the "handle" bit has now broken off, or if it originally just had a hole, there would need to be something similar to the "handle" part in the middle of that valve to locate this flexible flap.

Does this air which passes through the vent hose reach very high temperatures?  I've managed to remove it and it wouldn't be much troubl re-attaching the mushroom valve but I'm not too sure about if glue would be good enough to hold it in place if the temperature gets very hot.

I'd guess that any glue you could find would have trouble holding it in place as engine oil will have contaminated everything - the "event" being referred to would be any thing from a pop,bang or a boom - that is if it ever happened, best to fix this ASAP so as not to need to fret about this possibility.

OK thanks.