Given a theoretical combustion engine, how can I calculate the output of this engine depending on the fuel it uses?
The answer to this heavily depends on whether you have an internal combustion engine or an external combustion engine.

External

Because you're just heating the liquid from the outside, you can approximate the final output of the engine as the amount of energy gained from combusting the fuel multiplied by engine efficiency (as you like, but no greater than 100%). In other words:

$$E=kfV$$

Where E: Energy Output; k: Efficiency absorbing the energy; f: Thermal Energy obtained from burning a unit volume of the fuel; and V: volume of fuel combusted.

Internal

For an internal combustion engine, far more factors need to be taken into account. Wikipedia is always a good start to get a sense of the factors that play a role, and whether or not you want to consider them. De
pending on how dedicated you are, and how much the player can 'mess with' the settings of the engine, you could do something as simple as the External Engine (if all the user can do is select fuel and amount of fuel) or as complex as is required based on the variables the user can change.

Some Other Factors to Consider (Optional):


Compression Ratio (internal only)
Gear Ratio (for high torque applications)
Working Fluid (external only)
Ambient Temperature (mainly external only)
Heat Exchanger Material (external only)