This gives you an idea what's involved ,
LPG Conversion Ford Focus - Autogas Experts
But check this also ,
" Hi, I have a ford Focus 1.6, 56plate. (37,000 miles)
I was going to get it converted, as i have had a conversion before (but not on a ford)
However my mechanic said the he had heard that LPG on ford focus had problems because of soft valve seats.
(Although he isnt an LPG specialist.)
Is this true, should i avoid conversion, or if i do get the valve seat replaced with hardened ones.
Conversion probably a BRC system with Flash lub kit. "
He is right, they do have soft valve seats, so they would need a flashlube, but if its installed from day one its ok. It's quite a popular car on gas and you hear of very few of them suffering. As everyone in the industry knows about it and fits some type of valvesaver system."