yes john is absolutely correct on this
getting a truck certified as any rating other than its original rating is an expensive proposition, I've done a few of these over the years and depending on what type of rating you want to make the truck in question its generally alot of work.
If you want specifics as stated by OSHA you can check this out here: hxxp://w w w.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9828&p_table=STANDARDS
(you can correct the spaces and erroneous characters I've added to the URL to make the link work).

but I'm sure you may have already been here looking for your answers and these are just general requirements from OSHA that cover a complete range of equipment in a specific market area.

If your looking for details about how to perform the work?
It's fairly simple as to how to make a truck 'spark proof' or 'explosion proof' , ANY electrical connection or device attached to the lift has to be sealed in such a manner that it is air-tight.
All connectors, terminal exposed, panels exposed to the atmosphere that vapors can get to, any component that generates a possible spark has to be sealed up and made air-tight. All applicable signage/decals and literature for that rating has to accompany the lift and it has to be re-certified to the rating it has been altered to by a certified person or company.

If you check with your manufacturer you most likely can find that most of them have kits that are designed to convert lift-trucks into spark proof trucks.

But as john stated, it is a VERY expensive proposition to consider.