Zen Tuo is the second villain to attempt to kill members of the gang, after That's Snow Ghost (1970). In both cases, both villains use explosives in an attempt to kill them.
Scooby and Velma both understand Chinese- Scooby's even heard speaking it when he's heard arguing with the owner of the dry-cleaner. Velma claims she can read Chinese (and presumably speak it as well).
H&B Tuna Company is a reference to Hanna-Barbera.