Which model Williams? If, for example its the TurboJet 285 or the Sport Jet 345 then its powered by the Rotax Ace 903. That engine also powers the one of the SeaDoo's. So you could try them or perhaps Rotax directly? The larger SportJets also use Rotax engines, and the DieselJets use Yanmar's. You could search for local service shops for the relevant brand.