[suicideneil]

BB gun for machine gun... definately. An a revolver cannon.... hehehe.
I think a battery operated soldering iron operated via a remotely operated on/off switch would be simple to make. Then a small pump (pc water cooling system type) could be used to squirt diesel or someother liquid that smokes when hot on to the iron- use a needle from a nitro engine= ready made atomiser, and it would be adjustable too (a carb assembly could be easily modified= fuel tank-pump- fuel tube to carb with needle- sprays onto iron- lots of smoke). You might need a small fan to blow the smoke out of the dummy exhaust, since the real thing injects diesel into the exhaust like you said so the exhaust gasses force the smoke out- no such pressure in an electric model, so fan would be needed. I think that would work with a bit of jiggery-pokery to fit it all into the tank.