I did some research into this a couple of years ago, including whether there were any LPG conversions available for 4 stroke outboards. I was advised at the time that there were but since they were low volume production from CNC machined kits they were expensive but volume production was coming. Well to the best of my knowledge this hasn't happened yet!
The other thing that reall killed it for me was tank location - needs to be vented to the air / overboard which I think would make underdeck tanks probelmatic and whether the tank would stand up to the pounding that a RIB takes.
Having decided it was a non starter I then went on to look at Diesel (numerous threads about petrol vs diesel on rib.net if you search) but decided that purchase costs for what I wanted were out of my reach. Only option then is for hitech 2 or 4 stroke outboards and accepting that 1l per mile (approx) consumption at 80p litre is the price to pay for being a petrolhead!