Thsi goes back a little bit (8 years ago) but there is/was a company in Ohio that mad a CNG kit for forklift - the engine had to be an OEM LPG fuel engine. The Coversion kit was about $2600 & that is a SWAG. With all the Tier 1,2,3,4, etc maybe the systems can't be retrofitted due to CARB regulation. Do a Google search & maybe you can locate them & get more accurate info.
In my opinion CNG is overated for forklift operations. My rationale. A fuels energy rating is based on BTU (British Thermal Units). For example if we assign a BTU value of 100 to gasoline; then LPG would have a BTU value of 85% (less power out of the engine - less torque to the wheels). Well with CNG the assigned value would be about 70 (20% less than LPG ) & the unit performance is further reduced (some say not enough power to pull a sick person out of bed - had to clean it up a bit to maintain political correctness. There is less run time per tank full even though the tank is lmuch larger - more refilling required - If you need fast refills the refill station is a bit expensive. If one requires 5 trucks running on LPG to do the work, then you would need to increase the fleet size by 20% or one more unit to do the same work but also more emissions -hmmmmm. If one is a union shop or government run opertion that might be OK.
Other than that CNG is good.