Try prefacing your variables with "dec". This may not work, I've never used I2CWRITE before. There is a table with various data prefaces under serout, hserout, and lcdout commands in the PicBasic manual. I'm not sure why the same table wouldn't be under the I2CWRITE command unless it was not applicable.

I2CWRITE PortA.0, PortA.1, LCD, 0, [12, "Light: ", dec S1AmbientLight,13, "Range: ", dec S1Echo]