Originally Posted by MarcusT

Bosch is not an add-on system, they come as a whole bike only. You need to look for systems like Bafang to convert.
There are 2 types of motors for conversion.
The mid mount motor, similar to the Bosch systems
and
the motorized wheel, there the hub has the motor in it.
Depending on your budget, you can choose differently powered motors and battery sizes.
Youtube has some good tutorials