If you're lucky, the conversion parts are mostly still in place, and it
could be as simple as a replacement tank and a careful check of the
plumbing, especially around any coupling and the controller.  (You
definitely don't want a leak!)

If the van is fairly new, contacting the manufacturer to see if it was
a factory offering might be worthwhile.

I wonder if CenterPoint Energy could (and would, a different issue!)
tell you.   They're the corporate heir to Minnegasco... and one of
their other branches, Entex, is involved in compressed natural gas
(CNG) stations as a business that began with its own fleet.  See for
instance
http://altfuels.centerpointenergy.com/news/archives/090102.asp

Here is some background information that you might find useful:
http://www.pugetsoundcleancities.org/pdfs/cng_lpg_conversion_facts.pdf

(That page has a bit of potentially bad news:  that the most expensive
item in  the few- to several-$k conversion system is the tank.  Maybe
that's why somebody removed it!)  

Best of luck,
--Joe