Modifications
These are only some of the modifications that you can do to your spud gun.
Spark plug ignition
For simple spark plug ignition all you need is a spark plug, a battery, switch and wires. For this I suggest using motor bike spark plugs as they are smaller. Just simply build the following circuit and install the plug on the side of the pipe.
Fuel injection
For a simple way to refuel just take a refuel valve of the bottom of a refuelable lighter and screw it in the end cap. Then all you need is a can of lighter fuel and you don't even need to take the cap off.
More modifications when I get round to it.