>>1691830looks like 74 mm x 5 pcd Biopace chain rings of 28 38 48
the next smallest Biopace triple is 26 36 46
you could swap a round 24 tooth chain ring onto there but that may complicate shifting due to front difference of 20 teeth limiting you to 44 teeth on the largest chain ring
also your rear Derailleur may be a little ham strung in max sprocket and chain wrap
what might work is,
2002 Shimano Acera RD m340 (Capacity: 43T)
Sunrace CSM40 7BV 12-34T (or equivalent)
I dont know, ask your local bike shop
