All good ideas in the previous three replies - Especially about the brakes. I know its blasphemous, but I reckon the best micro for this application is a pic because they are far cheaper when you start adding in the on chip AtoD's etc that you need for throttle position sensors etc.
Anyway - enough of my opinion. I usually use a really simple round robin scheduler. Each 'process' consists of a single pass subroutine which is either called or not called by the main loop depending on a bit. i.e. if bit set do task and clear bit. Set the bits at interrupt level based on the appropriate external event. You can then put links on the board which determine whether the bit gets set or not to enable / disable each task or set of tasks. This shies away from what I think you want (pre-emptive multi tasking) but I've found that its good enough for all that I do.
Are you using inductive discharge, or capacitative for the ignition? If its inductive, do you want constant energy, or are you not bothered?