Do you mean the actual speed in rpm, or the value of your variable "motor_speed".
In either case you need to give details of your LCD: they aren't all the same, and imo the easiest to use is an I2C version. It's easy once you install the appropriate library and look at the library's examples of how to display a variable.
If you need the actual speed in rpm, that's more complicated and you will need a sensor like an encoder or a Hall effect sensor.