Missing screws are the easy part. Take one to your local hardware store and find a match. Many of them have a thread tester set up so you can get the size right (just a piece of plastic or metal with threaded holes).
It's more involved for the ones that are stripped. You're going to have to get the screw size, then get a drill bit and thread tap for the next screw size up (plus bigger screws). Like, if it's an M3 screw, get a tap for an M3.5.
You can get assortments of machine screw sizes online if you think you'll have a need for more in the future, but most big box hardware stores will sell a small pack of each screw size.



