I don't believe that you're able to buy singular screws on McMaster, so trying different ones may become expensive.
Btw, I guarantee that the threads on your screws are metric, and are most definitely a common thread pitch and diameter.
I suggest that you measure the ID of the holes, and look for a metric screw with a diameter close to that.
You can order small quantities from mcmaster. Though would be easier to go to a bolt and screw supplier that is local to you. Not lowes or home depot. I know for a fact HD will not have anything that small. (I work for home depot, and I order all the little packs of screws and shit on the hardware aisle.)
The bolt in the striker is different from the bolts for the Vsr. The Vsr has 2 bolts, one is electronic. In my opinion, the electronic bolt is better in the Vsr. If you want to spend money, buy that. It will work in the Ssg, little modification required for install. Nothing abnormal for a normal Vsr bolt install.
Converting an ssg10 to left handed is easy. Pretty sure not has the parts on his site.