I looked into partly substituting CNG or LPG for diesel in my previous car (Volvo V50 1.6D)
Wasn't economically worth it - savings are low, investment relatively high.
For trucks, diesel-mix systems are available, with up to 50% gas subsitution.
That system wouldn't have costed 50$ or €, but a lot more.
We can't drive around with DIY engine adaptations - yearly inspections starting @ 4 years.
Some research show environmental benefits, others don't when adding LPG/CNG
The right amount of gas (wether LPG or CNG) is essential though
In part-load situations, you may even see an efficiency loss ...
https://www.sciencedirect.com/scienc...10016815000162