I'm gonna split with Guy on this one.
The use of natural gas in a Listeroid may be because of need, so 'anything goes'. A lower power output is going to be expected, but the RPM's would be the same so the generator will still work, just less Kw.
The change to a 'dry' fuel may require top end lubricant, so you might need some (2 stroke?) oil to keep the rings and valve guides happy. Drip feed oiler? In the intake or in the intake valve oil pocket?
The combustion chamber poses all kinds of unanswered questions. DI head with spacers? Precup head with spacer (covering precup hole?) Would need longer block studs and pushrods. Original Lister head off precurser gas model? New head casting? Borrow design from precursor model? Or new tech combustion chamber? If anyone has done this already, c'mon, tell us about it!
Spark plugs and points have already been discussed.
Impco carb. With linkage to governor.
RA Lister & Co were great men, but they had their day with the CS. We can do our own adaptive engineering to suit the available fuel. Listeroids are the engine that has this potential with such small investment of time and money. Besides, it's fun
Scott E