I don't know if any of you folks follow the TerraTrike Owners group on Facebook. I find it an interesting group of folks. Yesterday during a discussion of a fellow's question regarding stiff steering on his Rover I posted the solution I found and Gary Greenway posted the thrust bearing upgrade that he implemented. Seems to me that it would be quite worth the while since it used bits readily available from McMaster Carr and the cost is only about $33 plus shipping. The necessary parts are....
2 ea 5909K15 Thrust needle roller bearing for 20mm shaft diameter
2 ea 5909K75 1mm thick washer
2 ea 5909K87 2.75mm thick washer
These are sandwiched together with the thick washer on the bottom then the bearing and the thin washer on top. The assembly then replaces the existing flat washer at the bottom of the kingpin. Suitably lubricated, of course.
Combined with the very good lateral support offered by the plastic bushings in the headset the needle bearing assembly should significantly improve wear resistance.