LPG doesn't destroy these engines - lack of maintenance does. Yes, LPG runs a bit hotter, but with a well maintained cooling system it doesn't run any hotter than normal. Also if the LPG isn't set up properly, then it can cause bad running, throwing fuel trims out, and can lead to rich/lean running all the time. Lean running can cause the engine to run hotter - but again, if the LPG system has been calibrated properly, then it will run with no ill effects.
Mine has been on LPG since 2008 - which is 4 years before I bought it. I put a new water pump in as matter of course when I bought it (the leak that I saw was actually the gasket, not the pump as I had thought). I put a new engine in mine back in January, as I had a couple of core plugs that were starting to leak coolant, and rather than take mine off the road for however long it took me to strip the engine - and have the logistical issue of getting to/from my workshop, I decided to fit a new engine that I built up myself. I ran it on petrol for the first 1K, and then started getting the LPG side back in with new pipes, injectors, ECU (the old one was showing signs of heat on the board), vapouriser, filter etc. It's now been running on LPG for the past nearly 5K and is happy. Whilst the engine was out and as part of the rebuilt, it again had new water pump (Quinton Hazell), thermostat (genuine LR), radiator (Nissens) and a new set of hoses.
I also did a summer meet up of a few P38 owners from another forum I'm on. out of the 6 that arrived... 5 of them were V8's on LPG. The other was a diesel. One of the guys who has a V8 on LPG regularly does trips to the south of france, towing a loaded car trailer - about 3K return miles I think he said. None of us have had the dreaded head gasket failure, or even worse the "oh you run on LPG so you're guaranteed to get a slipped liner" bollocks which seems to go around. I just laugh at people now when they try to tell me that LPG will kill my engine.. My old engine that I pulled out and sent back to the rebuilders didn't have burnt valves, blown head gaskets, or slipped liners after 8 years of running on LPG.
Once it's set up and running properly, there's next to no noticeable difference power-wise running on gas, and it's a lot cheaper (and cleaner burning usually) than petrol. I just make sure the cooling system gets a bit of an extra check over - and when I top up coolant, I make sure it's with a correct water/antifreeze mix - rather than just topping up with straight water.
Flashlube is a valve saver lube, which is fitted to some LPG installations where there is cause for concern that without lubrication the valves will burn out ( as LPG is dry and doesn't lubricate the valves, unlike petrol which does.).
That being said, the Rover V8 has hard enough valves/seals that you don't NEED a lubing system on them. Mine doesn't have one, and neither do pretty much every other P38 on LPG I've seen. I've heard the newer RR's with the jag engine have valves/seats made of something a tiny bit tougher than cheese - so with LPG on those, you need a lubing system or you'll burn them out..
My little pro-lpg rant over... and back to your issue...
If you have a system with injectors on each cylinder, then you shouldn't need a cat flap in the air intake - They were mainly installed as a flame trap for the earlier single point systems where it was basically a gas ring in the air intake, and a backfire could then blow the air box off and destroy the MAF.. The newer injection systems only deliver a squirt of gas into each cylinder inlet (as the petrol injectors do) when they're fired by the engine ECU - rather than having the whole inlet tract full of gas all the time!
If you've replaced the O2 sensors, and MAF with genuine Bosch ones not so long ago, then I would also look into the wiring and make sure that is all OK.. I've seen a couple of times a wire get squished on the Bosch ones, where the upper inlet goes on, as it is a bit of a squeeze under there, especially with extra LPG wiring.
If you are getting rid of the LPG - as I mentioned before - there are only a handful of connections to the petrol loom.. a power source, 16 injectors feed/return wires, a rpm signal, possibly 2 lambda sensor connections, and I think that's it. The rest of the lPG wiring goes to it's components - the tank/vapouriser solenoids, temperature/gas sensors, level sender, changeover switch etc.
Some systems have piggyback plugs for the injectors, which makes it easier to install/remove, but the others (if it's a decent install) should be soldered/heatshrunk into the original loom. On mine, they tapped into everything in the engine loom where if goes into the rubber sheath along the firewall. The other option is, buy a second hand Thor loom without LPG installed, and just swap the whole loom over, then you don't have to worry about trying to find/remove other wiring.
are you gonna tell me my high beams need oiled 
