Is there an affordable solution to add CNG or LNG to our diesels?

I don't think so.

From what I can tell, quite a few engine mods are needed to run CNG in (especially turbocharged) diesel engines, including lowering compression ratios and electronic fuel injection.

Its much easier on a gasoline powered car. But it still isn't an easy mod. Theres a lot more safety concerns with this fuel choice than say WVO/SVO.

Honda sells a CNG Civic in the USA, and you can get a home fueling station to compress NG from your utility company. However, the energy density is less and the vehicle only has about a 200 mile range. CNG isn't too easily to come by on the roads. I only know of two stations to fill at in all of metro boston.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car