This is what my dad uses. Gas burns hotter than petrol so I believe the converting mechanic has to take this into consideration and change certain parts, but I can't remember what.

Also just need to make sure that occasionally you put in a full tank of petrol and turn the gas off, run it on petrol with some injector cleaner. My dad's car started running terribly when it started on petrol, fluttering and reluctant. This fixed the problem.