Optimizations from a basic ignition-coil circuit:

• find a smaller flyback transformer (coil-on-plug coils?)
• rather than using a relay or any complicated setup, just have a largish capacitor that gets charged, and a manual switch that dumps its voltage into the primary winding (eg. if you don't need continuous mode, you can do away with the oscillator, etc... and the capacitor lengths the time of individual sparks)
• figure out some sort of compact high-voltage power supply to charge the capacitor (the circuit for a camera flash, from a disposable camera?)

Okay, these seem like the way to go.