check that your circuit opening relay is actually working, it operates the fuel pump in the tank, once while cranking and then the AFM latches it to ground to keep it running, I beleive that your CNG (******* mechanics and fitters) did not isolate the fuel pump circuit, and now is super weak.
This engine uses non adjustable bucket tappets, so if your valve clearances go tight - you need to remove and rebuild the whole head, (crack smoking toyota engineering at its best),
It may also be a weak AFM itself.. I would check the usual first, spark, fuel and compression - any engine needs these three to run..
no spark - dead crank angle sensor
no fuel - dead fuel pump
no compression dead engine