Chain Plate Replacement
By all means make them thicker and wider if possible. Normal 316 or 316L is fine. To help mitigate fatigue in the future put a highly polished finish on all the surfaces: sand (180 then 320 then 400 grit) then polish (Tripoli, etc.) to a mirror surface. No welds, no surface irregularities - no notches, bends, etc.
Larger cross section will give you more *load cycles" and the mirror polish will help prevent the micro crack formation. Fatigue cracks are additive and the more ''meat'' you use will provide longer service.
Dont wait as the price of stainless steel is rapidly escalating at this time. The longer you wait the more you are going to pay.