Does it run ok and idle smoothly on petrol? If not there's no way it's going to be ok on gas. LPG needs a really good spark so old plugs, dodgy leads etc are a none starter I would try the standard factory plugs unless they are the multiprong variety. If they are I would get a single prong one with the same heat range so you can adjust the gap. If idle is lumpy or it splutters when asked to run on low throttle openings under load at standard settings them I would close the gaps slightly and see if that improves it.
It took me ages to figure it out for the combo and it was only because I stumbled across a listing online that showed a different gap for the LPG models I sorted it, that was coil on plug too so should have been a fantastic spark.