Jason's Wizard Mercury comparison chart on the main website says that the WB3 is comparable to the Mercury K2, KB2 and KB3.
They all had the same displacement and cylinder size, but the latter two were 3.2HP compared to the K2's 3HP.

K2 weighed 28 lbs.
KB2 weighed 32 lbs.
KB3 weighed 36 lbs.

Some parts here:
http://store.oldmercs.com/category_s/12.htm

Sometimes Ebay has used parts, but it takes patience.