I would recommend replacing the entire propane system. This system regulates a flammable gas & if it's malfunctioning in one aspect then the entire system is compromised. Unless you know for sure that the system has been serviced rercently (within the last month or so) , to avoid future headaches & possible damage to product or operator I'd replace the entire system.

I would replace the Impco model J convertor with a Impco Cobra model though. I've had lots of problems recently with the model J convertors on the market. Replacing the diaphram in the carburator would be sufficient (don't have to replace carb), and the VFF-30 replaced as a whole unit would complete the fuel system. These parts together shouldn't cost more than $500 from any dealer in your area.