Did you try Ravelry? They have patterns for everything and you can check for just knitting patterns. Honestly though if you knit you can crochet with no problem! I crocheted first and learning to fumble around with 2-6 needles is way more challenging
I feel this would be fairly easy to adapt from existing patterns, too - a close fitting cap with a bit added for the neck, maybe a large i-cord with pipe cleaner inside for the bit on top and a triangle with straight centre panel (double layered) shape for the mouth guard. Maybe play around with some triangular shawl shaping for that (obviously smaller number of stitches)?
ETA - found these patterns that are kind of similar on Ravelry. The last one is very close to the picture you posted and is kid sized!