Get a different ecu with triple map and run LPG on a cossie with whatever power you want.
Use ULTRASEAL and you don't even lose boot space, as no spare needed then, you ca even gain by removing spare and the foam piece above the spare wheel.
LPG in "certain" places as little as 38p litre, the only difference is it does NOT have a STENCH agent in it, as used for HEATING, so if you have gas heating maybe a switch over!!
The tiny amount of mpg lost is more than covered by oil that does not need changing so often.
My LPG xr4i used to do 25,000 miles between changes, and it was pretty clean when changed, as no carbon in it like you get from liquid petrol.
They do tend to run slightly hotter, but plugs also last a lot longer, they are MUCH MUCH smoother, will start from a frozen winters day cold start no problem(system dependant), are quieter too, you can smell a tiny lpg smell at times but only slight.
If you did not want to go different mappable ecy type you can get a AEB wolf "N", which is a spark advance unit, just for when on lpg.
tabetha