Thanks for the reply. I take it you are talking about the 'normal' zetec engines. It's good to know there are factory pre-converted versions of this engine because that means the parts are at least available.

But because the ST170 engine is a modified version of the 2.0 zetec engine, its my hope they modified the valves in such a way they can handle natural gas.

I found the exact ST170 engine specifications on this website: http://www.fordst170.co.uk/specs.php

I have insufficient knowledge to determine the engine can run on natural gas based on those specs, but perhaps someone else on this forum can.