I suspect your cranks are the common 3-arm style. These had a bolt circle diameter of 116 mm and this was virtually an industry standard for many decades. And, 3-bolt chainrings between brands were typically compatible and interchangeable. You might easily be able to find good chainrings in 42t or 48t sizes if you could work your rear cog around those gear options. If so, you could simply remove one chainring (and possible a tacky outer chainguard ring) and just use a single ring on the bike - just left basically as is.

Also, if you can find a square tapered spindle of a suitable length, you may also be able to still continue to use the existing cups. Manufacturers were always happy to go with their cheapest options and minimizing their parts stockpiles. So, the cups may be virtually identical with cotterless cups - even on the insides. In my experience there was often adequate clearance in the axle holes of the cottered cups for slipping in a square tapered spindle, and the bearings and cones of the new axle will likely be perfectly compatible with the cups too. You might just want to check this out before embarking on a quest for some excessively elusive and expensive replacement parts, which you may not even need.