this is going to be a tough nut, sterlings were marginal at their best.... and horrible at their worst. they were disposable guns, much like the jennings/bryco/jiminez/raven/davis guns are today.
IF you must use this gun, you may find a local machinist who can make the screw for you, but it will most likely cost more than the guns worth.... one other place might be ACE hardware, they have a crap load of machine screws in all sizes, take the slide off and take it in for a test fitting. if you get the right thread pitch and shape, you can always shorten the screw to fit.
IF you dont have to use this gun, i recommend that you piece it out and sell it, in "like new"
condition its only worth $90-100.