Hi Shaun its GAS all the way apparently according to the makers ( wisconsin) the engine is fine with it, And I have has the same set up on my generator that powers my place thats off grid, so I'm hoping that it gas all the way.
The Difference is that the engine is air cooled so it needs a vapour lpg tank to run it ( just like a regular propane cylinder ) And that has took some effort to find a certified one.
Where as a water cooled engine uses liquid propane feed to a vapouriser that is heated with your engine coolant ( hence it needs to get warm first )
Would have minded doing your way but I had to do it this way as the burner is also vapour feed, so it would have meant two tanks, must admit I'm kind of interested myself as to how it goes when I finish, the Maths is right by lets face it Theory is never the same as practice when it comes to carpet cleaning.