Its not hard to convert an outboard to run on gas, we have been playing with LPG outboards for a long time. It has to be a 4 stroke though!. We have just finished a Mariner 60hp EFi running on LPG and a 20hp. The biggest problem is the rules and regs about having lpg tanks in a boat its a total nightmare!!. Its also very hard getting all the lpg stuff under the cowling!.
The LPG tank must be vented from the bottom of its locker over board, so on a rib its very hard, the gas must "fall" so the outlet must run down hill and vent outside the boat. So unless you have gas tank sat in your rib at waste high its pretty hard to do. The LPG must vent above the fully laden water line on the boat too. You cant use a blower either by the way!. We think we have cracked it and are currently having a 4.8m Ribcraft built as a demo boat which we are hoping to have in the water a the Seawork show in June if anyone wants a ride!. We looked at putting the lpg on the deck and venting it through the bottom of the transom but the tanks are pretty large and take up to much room in the boat, also the weight of the tank sat at the back of a small rib causes problems with weight. There are rules about having the tank inspected so the tank must be accessable, so you cant have it mounted under the floor ( this would also stop you venting it as the gas must fall to the outside of the boat) LPG being heavier than air. You must also have expensive gas lines made up. You cant run LPG fuel lines under the deck as you can see them if they have a leak. Its all very hard work!!. This is just a small part of the nightmare rules and regs which is why no one makes an LPG outboard. Mercury bought out a small range of engines but dropped them as it was to hard to comply to the regs. The regulations are all set by code of practice 18 set by the Britsh Marine Federation, Recreational Craft Directive if you have insomnia and fancy a read!!