There's a general equation that usually works well.
[(Teeth on top gear )+(Teeth on bottom gear)/2]+40 ... round up to the nearest even number.
(21+42)/2=31.5 ; 31.5+40=71.5 ; Therefore a 72 pitch chain should fit about perfect.
I believe those ran a 13wide chain. You should be able to count the links going across the width to be sure.