1. Engine volume increase
The most common way is to increase engine displacement. Thereafter, the engine proportionally increases fuel supply. This is quite applicable to old BMW engines. Engines m40, m43, m50 and called indestructible for this reason. Their life with proper maintenance is really very big.
Power increases in proportion to the volume. This can be achieved by increasing the diameter of cylinders. But at the same time greatly reduced service life due to the increase of thermal load on the parts. Really, you can encrease cylinder's diameter adding 1 - 1.5 mm.
2. Chip tuning
Tuning of the engine control program to obtain the best power characteristics. The power gain will be small. It can also be safely used with BMW E36 engines. The service life of engines from the factory allows without danger for the engine to raise theirs power by 3-5%. All that you need is visit a cool servise with right computer program.
3. Use of turbocharging.
This is the most competent, but also the most expensive way. First, the installation of the turbine requires the installation of a number of additional elements and therefore is expensive. Secondly, it greatly raises the power of the engine, which requires a refinement of many systems of the car, including the engine itself. This type of refinement engine reasonably be applied to the most advanced bmw e36 engines m50b25 and m50b28. For smaller engines this is not the optimal way. It is better to use the SWAP.
Turbocharging really can add up to 25% of the power of atmospheric engine.
Often the simplest and best solution is the installation of a new engine with a large volume and, consequently, a better dynamic performance. It's called SWAP. You'll need install more other car parts, not only new engine. For example, you'll must upgrade the transmission (clutch, cardan, reduction gear), braking system, intake system, the motor management programme and so on.