Replacing the stock narrowband O2 sensor with a Wideband O2 Sensor and Controller may be more of a project than many would undertake. The more I've worked with it the more comfortable I've become in saying that it's quite straightforward and without complication. It's also possible though that you could get a modest shift of Lambda with the stock narrowband sensor.
If you go back to page 1 of this thread you will find the chart that I'm reposting below. This is a plot of all the O2 sensor voltages that the GS-911 acquired during an extended test run. It is that known that something in the vicinity of 450 millivolts (0.45 on the chart) corresponds to a Lambda of 1 (meaning an AFR of 14.7:1). What is not so well documented is what is the AFR on an Oilhead when the O2 sensor voltage is between 650 and 900 millivolts. If the AFR is only 1% richer in that range then there's not much that can be done with the Narrowband sensor. However if those voltages correspond to a mixture that is 3-4% richer then there is probably a circuit and set of bias voltages that would allow a slightly richer operation of the Oilhead with the stock O2 sensor.
Since I'm planning to have the LC-1 initiate a recalibration of the Wideband Sensor (another neat thing about the Wideband O2), I'm thinking of putting my Narrowband in temporarily to see what the Motronic will do with a modified sensor. In other words will it hold Closed Loop if I alter the biases and voltages?
If that worked, then I might take my exhaust to a welder and have a second bung added so that I can accurately measure the Stock O2 sensor under standard and modified conditions.
If anyone knows of a good quality welder in the Metrowest Boston area I would gladly take the recommendation.
Stock O2 Sensor