Obviously, any type of indication that the wheels are turning will do the trick for the spark enable signal. I'd be thinking of something a lot more random than a periodic timer pulse for the sparking that will not really look that realistic. I'd consider something like the flickering LED's in blue or white (or a combination of those) to generate the "sparks".
Perhaps an even better solution is the ngineering.com lighting effects modules. The page is Lighting Effects for Streets & Highways, about halfway down the page.
