M20 engine's not like a 350. Its an interference motor which is why if you blow a timing belt you stand an excellent chance of detonating the motor/valvetrain. The old hot rodder trick of installing the cam a few degrees advanced or retarded doesn't compute to the sohc inline 6 here. Engine timing must be stock if you expect your engine to last (or function really). The flip side of the coin is you can play with the ignition timing per chipping the computer. Its fast and easy and you don't even get dirty doing it. BAV auto and more than a few aftermarket beemer places have chips and such to do what your looking for. Beyond that you'd be looking at a scalable nitrous system, or turbo/supercharger. There's also a popular engine swap to an e36 drivetrain if you want to get really nuts with it. Of course at that point you should seriously just look at breaking down and buying an "M" car.