You can get the 1/4-28 button head screws at your local hardware store. If you don't find the exact length just cut them to the length you need. The rear screw length is not fussy but the front one if too long will hit the bolt head. You want to just clear the bolt head to take advantage of all the threads in the reciever.

Use steel wool to get the rust off the mag spring. That rust happens, is normal and probably won't hurt a thing.