If the engine only runs with the choke partially pulled then the LP system needs service. You are correct that the parts for your system are getting harder & harder to get- upgrading to an Impco system really is your best bet. Newer versions of the C500 series came stock with the Impco system so converting it to that system is fairly easy.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.