I have an old Hastings Catalogue (No relation?) and for the JXD engine they list 2 sizes of Piston rings for JXD engines (4.00in bore plus oversize) :
Up to Ser No 11462: 18x 1/8 rings, 6x 3/16
From Ser 11462: 6x 3/32, 6x 1/8, 12x 3/16
As these are 4-ring pistons, I doubt you'd find a complete off the shelf kit from a Ford 302, Chev 350, Holden 308 or Chrysler 360 (all 4.0 bore and 1/16, 1/16, 3/16 ring sizes)
I'd give
JP Pistons in SA a try.
Another alternative might be Ross Prince in Qld.