Punter, inspired by popular mechanics I looked into Sprinter cng injection.
These are dual fuel kits, you start on diesel and run a percentage say 50-75% cng and diesel.
If you bought a 25 gallon tank and all the parts from technocarb $3,000. The most critical part is to program the piggyback ecu.
CNG interstate in Utah would sell me their kit with tank and program the ecu for 5,800.00 or a turnkey install for 8,900.00 ftb, Utah.
You could compress your own gas off your home gas line, which would be really cheap, but the cheapest cng compressor takes 20 hours for a cycle and costs $3,000. Here in So Cal cng is 2.50 a gallon
, in Oxnard and to the north $1.00.
An oem cng motor would have hardened valves and seats to cope with the dry nature of cng, spark plugs and a lower compression ratio. Also diesel is an important lubricant in our motors so you may need top end lubricants to make it live.