The conversion is not that hard to do. There are bolt on kits out there. Go to the site timcasbolt posted. They will need to know what engine you have. Full model and serial number.
You will be dealing with a vapor system. Liquid withdrawal systems are mostly used on Water cooled engines. Because you need a special vaporizer to turn the liquid to vapor. They do use them on air cooled engines. But not that common.
To shut the fuel off. You can buy a solenoid that will shut off the fuel when the engine shuts down.The folks at the carb company can explain it all to you.
If you need more help. Check out smokstak.com Onan section. Plenty of guys over there that have done the conversion. They are all willing to answer any questions you have.
Onan tech since 1980