P38 (LPG 4.6) - Do I need a new engine?

It's not done badly - 170,000 miles on a 2002 - but is beginning to show some faults. I have a bad misfire - which new HT leads, and plugs didn't fix. One of the plugs showed rust - so I thought water was getting in. Garage says it could be the plug sweating if it's not firing properly - but the tests showed hydrocarbon in the water after 3 minutes - indicating some damage to the engine. It doesn't lose much water, the oil is clear, and there is no pressure or bubbles in the coolant tank. One garage says I need a new engine - the other that it is probably OK, and that the misfire is probably un-related. Any ideas?