I carry a few different sized rawl plugs which I use when this happens. Another way is to use a thicker screw. Top tip...find a spare or repair oven and strip it down. Put the bits in a box and keep it with you. They'll be a lifesaver in the jobs to come.
As what @Richard Robertson says, carry some thicker panhead screws. Living on the coast, I have a few boat supply (Chandlery) shops who sell an assortment of loose screws which means you don have to buy a box. Sometimes the hole that the top element screw goes into is too big to get any screw into. I have used this type of rivnut/spidernut which allows you to screw an engineering screw into once fitted. Also you need to point out to the customer that there is a rust issue and charge for a repair. You will need some tools to ream the hole to fit one of these into and a nut, bolt, washer with a couple of spanners to make the nut spread around the hole.