Vintage bike - help with chainring replacement
I believe I have that same crank on a roughly 1983 Schwinn World Sport (see image). I got it used, so I don't know if it was the original, but in any case, mine has 40 teeth on the small ring and 52 on the big ring, so that's likely the size you're looking for. 

That said, it might be easier to find a replacement for the entire crank+chain rings rather than match that chainring. Mine is a standard square taper spindle, and many vintage cranks that would fit are available on eBay etc. 

If you live in a city with a co-op that salvages old bikes, they often have loads of cranks from that era that would fit. 

Furthermore, since the square taper spindle was a standard for so long, a more recently made crank is also an option. 

UPDATE: Images online seem to indicate this crank was standard on early 80's World Sports. Given how many of those bikes are still around, which means the cranks should be relatively easy to find, or you could easily find a whole bike and take the crank off if it's really important to you to match your original.