How can I fix this portion of sliding screen door track?
This is a tough one, as the aluminum is hard to weld.  And replacing the sill plate is tantamount to replacing the door (strike one for planned obsolescence).

I'd first try trimming a small scrap of aluminum slightly wider than the hole.  Roll the top edge so it takes the weight of the door. Then clean the heck out of the area, and epoxy it into place.

This has a chance of working.  Don't bother trying to restore the weep function, you need all the strength you can get.   Don't worry about making the track slightly wider, the roller wheel is pretty sloppy.



Not used to working with folding metal? The top edge of an aluminum sardine can might give you a head start.  The top edge of a tin can would be even stronger at the slight risk of galvanic corrosion.  Sheet aluminum is available at any hardware store in the roofing area. JB weld is a fine epoxy.  I'd stay away from fast epoxy and make sure to clamp really well.  Remember you want the metal to take the weight of the door wheel, the epoxy is just to keep it from falling out.  Feather the edges of the epoxy to reduce catching.