Biopace 52x42 is almost certainly a 130mm BCD crank. Biopace was also made for 110mm BCD but those were typically triples with 48x38x28 or 46x36x26.

The smallest round ring for a 130mm bolt circle is 38. I believe the smallest Biopace for 130mm is 40 but you may not be able to find one.

If you get a smaller large ring, you will probably want to lower the front derailer - about 2mm per tooth reduction (e.g. if you go from 52 to 50, lower the front derailer by about 4mm) - or you may have poor shifting.