You might want to try Bob Grubb (retired owner of Grubb's Marine). He still maintains a used Mercury business and sometimes comes by Wizards. If he doesn't have any, he might be able to point you in the right direction. His e-mail is firstname.lastname@example.org.
I am not sure of the cowls are interchangeable with any Mercurys from that year. The WG4 is based on the Mercury KD-4, which was sold in the '40s and doesn't look exactly like the WG4. Again, Bob might know if there are other options for you, as finding just Wizard cowls might be difficult.