You posted 12/86 as your jetting, I assume that is your gear ratio? I am curious as to what was your largest main jet that you tried? Pooping is generally a lean condition that happens before a lean stick or failure. The K&N filter is a radical change in airflow to the motor which I haven’t tried. Just changing to a big main jet may not be enough change for correct fuel/air ratio.
Many other settings like the float level and weight, the needle and slide position and sizes, emulsion tube, even the expansion pipe and fuel pump delivery could come into play in order to get the fine balance of air/fuel ratio it had with the stock air box and other stock parts.
A crude test would be with about a 170 main jet then flip the choke on if it still poops to see if it runs a bit better. But your at risk, as with even a stock set up I have had just my float drop set wrong that caused it to run too lean, (bowl empty’ed) no matter how big of main jet was in it.