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
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Here is some background information that you might find useful:
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(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