As others have stated, more information is needed - i.e. amount of power,
cost of NG supply, etc., etc.
One option for long term durability in a much lower tech form would be an
oilfield cathodic genset. These are large bore, single cylinder, low RPM
NG engines combined with a matched genset that are designed and built for
long term, low maintainence operation. Typically used to power the
cathode protection on NG and oil transport lines in remote locations. The
engines themselves are used to operate pumps on wellheads in remote
locations, running on NG supplied directly from the well. Technology has
been around since the mid-20's...
http://www.arrowengine.com/en/liter...-series?orderby=dmdate_published&ascdesc=DESC