I think a generic assortment is a waste of money. If the assortment is not specific to Lionel products, it will probably not be as usefull as hoped.
I would take samples of screws I need to the local hardware, and see I could find duplicates and build up my spares that way. maybe do a few sizes per month or something.
My hardware stored used to be willing to order special screws for me. The store is still there, but I have not tried to do that for about 10 years. But they did it then. I just had to order a "box" which was usually 50 or 100.