If you want to use a 8051 compatible to take advantage of all the available info & free tools, you might want to take a look at Dallas Semiconductor's DS89C420. It is advertised as a plug-in replacement for the 8051 but executes code at 50MIPs(peak) @ 50MHz, giving you about 100000~200000 instructions per spark event period of 4 ms. This might be enough for your algorithm.
The chip is selling (in Spain) for about 20 US$, and you may request a free sample from Dallas at www.dallassemiconductor.com
Bear in mind that you must provide external D/A, as the chip doesn't have any onboard.

I haven't used the chip myself yet; I don't know if you can use a standard programmer to load its Flash rom...

Best Regards

J. Ruiz