I had a core plug fall out of a 90 turbo deisel, I am a boat builder and I use epoxy resin all the time, I made some epoxy paste and gobbed it in the hole, worked a treat, you need epoxy resin (not polyester), some chopped fibres and some colloidal silica, mix the resin then add some fibres then add the silica untill it has the consistancy of cream cheese, clean and dry the hole thoroughly, then gob it in. You need to be carefull as the epoxy goes soft when re heated and needs to be cured at gradually higher temperatures until it remains hard at 80 degrees. google fibreglass supplies for where to get the stuff, remember polyester is no good, hope this helps