The code below is the HelloWorld example that comes with the I2C LCD library, or at least the library I use.
Instead of the line:
lcd.print("Hello, world!");
use something like:
lcd.print(motor_speed); //with no quotes
at the appropriate point in your loop().
And of course stick all the lines from above setup() into your sketch, as well as the begin() in setup().
(Of course you might have a different library.....)