If it were simple, it wouldn't cost $5000+. It would involve custom porting and different valves - a lot of machine work. BMW doesn't leave a lot of power 'on the bench' in their engines like most American engine designs, so slapping on a set of headers or changing an intake manifold on a 'strangled' head makes almost no difference. Without the extensive head work, the biggest power differences will come from performance chips and cams. If you want specific guidance, I would suggest you contact T.C. Kline and/or Steve Dinan and be prepared to start paying from the get-go. Neither one of them is likely to spend a bunch of time telling you how to do it when they get paid to do it.