I think the likelihood of getting an identical switch to the one that fell out and has since become lost is quite poor. The chances that you'd just be able to pop it in as well are next to nothing. Even trying to find an image of this PSU with it attached flummoxed me.
It's more than a little dodgy working with PSUs. The capacitors inside PSUs carry massive voltage and can release them in an instant down an unspsuspecting finger or tongue, which would give your heart quite a flutter. I've blown a screwdriver blade apart on primary side mains voltage capacitors once and I'm so glad I was insulated from it.