Just take your existing filter off to test, it's virtually the same as K&N then anyways! Oh... as you are on LPG, K&N filters are NOT recommended on a draw through system, they do not allow the engine to give enough suck at the mixer ring, therefore weakening the mixture substantially.... cue backfires!
Try blocking off the exhaust pipe exit if you think that it is too free flowing for the engine, this should make it run better if that is the fault...
TBH, I'd put money on a wire or two being knocked off during the exhaust installation, as it runs OK on gas, you can discount any of the ignition system, and I'd start by looking at things like your fuel pump feed wire, lambda probe (assuming you have one?) and also all of your LPG installation wiring, it MAY be that you have gas and petrol going into the cylinders at the same time, not good!
Hope that helps!