Mechanical to electronic cattle scale
You could affix a force meter to the end of the weight slide, and calibrate it with a known weight. In theory, so long as you don't move the adjustable balance weights after calibration, the force meter would be correct.

Things to consider: 


Determine the max rated weight of the scale, and calculate the maximum pull force on the slider arm. Choose an electronic force meter accordingly.
Alternatively, you could mount the force meter equidistant from the fulcrum, then no calibration would be necessary, though you probably would have to take the sliding weights off.