Not sure how he did it, but the fact that it was a full replacement not a dual fuel was a factor from what I remember. Completely removed the petrol fuel system and tank. This would mean the above mentioned dual fuel issues would be irrelevant. You only have to start it up on petrol if the plugs are set for petrol. You can set them for gas so it'll start with gas.
PS I'm no expert - these are just bits of information I remember. We supply LPG at work but for fork lifts. If anyone wants any LPG parts I can get good prices.