From some preliminary research, I think that you can tune a propane carb to run on low pressure natural gas (many people make tri-fuel propane/gasoline/natural gas kits for generators). The conversion will run alot more than $800, however. The propane carb is over $300 (I haven't found a bi-fuel kit for mower engines yet--which would be more practical). A small, less than 3 gallon gasoline equivalent (gge) 3600 psi natural gas tank is at least $350 and weighs about 100 lbs (lighter ones in the 40 -50 lbs range are available, but cost 2-3 times more). Then you will need special mounting straps not to damage the tank ($150+), a 3600 psi valve (over $100), a high pressure regulator (over $100), low pressure regulator, pressure gauge, fill valve and safety, lots of hoses and fittings, etc.
Then, to fill your tank you will either need a commercial natural gas fueling station (which may prohibit the fueling of your mower) or a home refueling appliance (HRA) which will set you back at least $4000 plus installation and may only be able compress only about 8 gge of nat gas per night. Next generation HRA are in development (2 year eta) and should be cheaper.
Natural gas burns hotter than, which might be a problem in mower engines. Engines speced for it have special valves. Similar stationary small engines burn it, however.
I'll revisit the idea of CNG when the new HRA's are released. It is interesting because it could save about $4000 in fuel costs over the life of a mower (2000 hours x 1 gal/hr x $2/gal) at current gasoline and CNG prices--even more if gasoline increases.
It looks like Dixie Choppers CNG mower was not a commercial success. They are not listed for sale. Did anyone buy one, or where they just a concept mower?