It Would probably be cost prohibitive to convert a diesel to CNG as opposed to a gas engine. There are too many systems not present on a diesel that are required to run a CNG setup. I also doubt that you'd have adequate means to get controllabl diesel effect ifnition with CNG vice diesel If your application permits, the gas engine swap with CNG conversion is the best way to get you on the gas. There are several conversion kits out there with broad application listings.

Substitution of diesel fuel by CNG in Urban Buses (and trucks) is particularly advantageous because these vehicles on one hand have high specific fuel consumption and high mileage, and on the other hand, need only few centrally located filling stations in view of their limited range of operation. the economic and ecological benefits of CNG driven urban buses are illustrated by the following examples:

The conversion of one diesel bus to CNG saves about 145 barrels of petroleum annually at average operating conditions. Given a petroleum price of US$ 20 per barrel, this results in foreigh exchange saving of US$ 2,900