The OEM oil filter for the Westerbeke 30B is #WEK036918. Not familiar with parts availability in your locale; may have to order from an online retailer. Can also use a non OEM filter such as WIX; they have a solid reputation in oil filter applications. If you’re going to order online, may as well order other parts that need replacement, as you don’t know when they were last changed and the boat hasn’t been used in two years. Consider parts such as raw water impeller & gaskets, engine mounted fuel filter & gaskets (secondary fuel filer), primary fuel filter (between tank & lift pump), and replacement belts for water pump & alternator, to consolidate shipping costs. Good idea to keep spares for these parts on board. I use Shell Rotella T motor oil ( available at Walmart) in viscosity rating that my engine manufacturer recommends. Should consider flushing & replacement of engine coolant & check age / status of batteries.
Congratulations on acquiring your 331; nice boat!