Converting power and speed of a bike axle
My initial thought was to use an DC motor as generator to convert speed to an voltage we can read(with an power resistor to dissipate the power produced)


Yes, go with that. If the generator is large enough, and the power resistor is large enough, they can dissipate all the power produced and there is no need for a second motor as a load. 

Filament lamps make good power resistor loads. They dissipate a lot of power by being able to run hot, and they give a nice built-in indication of power! A useful side effect of their temperature dependent resistance is that they tend to a constant current load, meaning that the back torque they'll produce in the generator tends to stay more constant with varying speed than that produced by a fixed resistor.

There are several ways to vary the load with resistors, which range from switching several in and out, to using semiconductors in a linear way, to PWM'ing with semiconductors, to interposing a DC-DC converter between the generator and a fixed resistor.

The current drawn from the generator will be more or less proportional to the torque produced. The voltage from the generator will sag somewhat with the load current, it may be adequate for speed indication, or you could use another tacho, or small generator, or optical angle encoder, or an optical beam break, or a magnet and reed switch, there are just so many ways to get a speed indication.