Mitsubishi Challenger 2000: LPG Backfire problem

ONE of the reasons we bought our Challenger in 2000 was that Mitsubishi was the only maker of this size four-wheel drive to give us a guarantee in writing that fitting LPG was OK. We had the dealer fit the Impco system. From time to time we have had a small backfire that has dislodged the aluminium grille, just in front of the air sensor. Our local Mitsubishi dealer just refits the old grille. Their price to replace the system is $860 as it is not supplied separately from the whole metering device. We consider that far too expensive for such a small problem. Is there a cheaper alternative to the factory part we could use?