well, if you don't find the mag button , it'll cost $10-20 to replace it I'd guess.
Here is one;
http://www.gunbroker.com/Auction/Vie...?Item=85763121
I would think a mag without the button would sell for $30-$35 and with it, in the shape you said, $60, nicer it is, than $85-$100
You have to push the mag button in (it should be missing the button AND the stem), if the stem is still there, well, you're screwed, and i doubt you could weld the little button on. If you have the stem and button, then you brace one side and pop in the stem and button. I think you bring the follower down to the hole in the side of the mag. I can do it if it is in front of me...
Ed