The /z/ sound is spelled with <z>. The /j/ sound is spelled with <y>, <ew> in ewe or <u> in use. The contrast can only occur initially.
This is a contrast between an alveolar sibilant fricative and a palatal semivowel, both voiced. It is not a problem.
The mean density value is very low at 0.1%. The list makes 6 semantic distinctions, a loading of 55%.
zap yap zapped yapped zapping yapping zaps yaps zee ye Zen yen zoo ewe zoos ewes zoo U zoos U's zoos use
John Higgins, Shaftesbury, November 2010.