To replace the screws with socket heads...use 5-40's. Most of the tuners do. Any industrial fastener store should have them or be able to get them in short order. If you want, McMaster-Carr will work. Once replaced, they'll outlast the engine. Apparently, the 3mm x .6mm "standard" that ST and OS used in the '60's is no longer standard...3mm x .5mm is. Most .60/.61's are .945" bore, so most any of their head gaskets will probably fit. I'd look for OS .60FP or K&B .61 stuff. Fox .60's were .906" bore, so probably wouldn't fit. Anybody can make a backplate gasket out of almost anything, like a 3 x 5 file card, manila envelope, or plastic milk jug.
If you want genuine stuff, ask Randy Smith, Tom Lay, or Frank Bowman. I have email addys for all of them, and can probably dig up phone numbers for Randy and Tom. Frank has a "forum" down in the Vendor's secton...you have to login to see any of them.

Steve