A replacement gap piece may well be unobtainable. They are usually fitted and finished along with the bed, with perhaps a scraped surface as well which makes the piece individual.
+1
The gap piece is scraped in, fitted & pinned to the bed casting before the bed formation is milled, heat treated & finish ground.
If anything is available (which, for a lathe of that age from a builder who no longer manufactures, is very unlikely) it would be a part finished casting that would require fitting to your bed casting & then machining through (bed regrind) to get proper alignment. If you were to find the gap piece from another similar model the same would apply.
Could you check back with the people you bought the lathe from to see if the machine's original gap piece is sat under a bench somewhere ?
Nigel B