old plumbing trick , can sandwich pieces of rubber either side of the old hole, backed with some thin sheets of metal
then using some nuts and bolts to secure it all, first putting some silicone on the rubber as u squeeze it up tight with the nut and bolt
reminds me from my plumbing and heating days, using pipes , glands, fittings etc to get a waterproof seal
but like u say ref kits etc
not sure how the breather pipe is supposed to be set up, wonder if kit instructions may shed some light or another forum member
don't think ur too far away from me, but if u ever need a hand plse feel free to give me a shout
always good to meet other forum members and lend a hand
hope that's not being disrespectful in any way