I assume you mean the thrust bearing as used in the clutch? If so, these can be replaced by a sintered (not solid) bronze thrust washer quite successfully. They are sold under the brand name 'Oilite' here in the USA and are ofter supplied pre-impregnated with a light oil. They have the added advantage of being more tolerant of running up against a surface on the clutch sleeve and the throwout cam that may have been torn up and tracked by the original ball bearing thrust washer.
Buy a sintered bronze washer 3/16 of an inch thick (cost about US$9) and machine the outer and inner diameter to suit. Try not to machine the thrust faces if possible, rather buy a washer pre-made to the proper thickness. The reason is it is quite easy to smear over the oil bearing pores unless the turning tool is quite keen. This is of little matter on the inner and outer diameters as they do not see any significant bearing loads.
I have also used Torrington needle thrust bearings on the big twin OHV clutches, which are even cheaper than the sintered bronze washers. But offhand I do not know if they are readily available in a size to suit the 350cc EW, and they do need a good, hard, and flat surface to run up against. They are also thinner, and you may not be able to get one thick enough for that diameter.
All in all the sintered bronze washer works fine unless you are going to spend a significant amount of time with the engine running and the clutch dissengaged. At which point the original ball bearing thrust washer with its tolerance for insufficient lubrication would be superior.
-Doug
