Hi Hardyferret/pig,
I have used normal unshielded plugs in a "B range" engine belonging to a customer, cannot remember what it was at present, but I used the same plug as specified for the fire engine applications, which I think, is Champion N8, but will have to check that out first. The spark should be better as the screened sheilding produces an effect that weakens the spark, Clive will remember the term for it. I have done this on other screened vehicles and the result is so much better, especially if using plugs like Champion Gold Palladium or NGK Copper Core, because they have a wider heat range and do not foul so readily.