There is nothing electrical tied in with your mechanical speedometer. I could be wrong, but I'm pretty sure these engines didn't use speed sensors for any type of engine management decisions.

Code 13 indicates a fault in the oxygen sensor circuit. It does not mean, "just replace the sensor". It means the circuit is open, or the values the computer is recieving are out of acceptable ranges.

You could change your 02 sensor but that might not be the problem, run a quick diagnostic first -- it only takes a few minutes. I orget the exact procedure but it involves a digital multimeter.