hi ian,
i think the spec for simplex gudgeon pins is overkill, and certainly not what martin evans would have done himself or what most miniature loco builders would do.
ordinary free turning BMS if a good finish is quite ok with bronze small end conn rod bearings. these bearings wear quite badly if lots of heavy use as heavily loaded compared to size, plus usually receive insufficient oil, as difficult to apply.
my own personal practice with the few walschaerts locos ive built is to case harden the gudeon pin after making with a nut on the thread. i use hard drawn phos bronze for the small end bearing (dont use gunmetal). ali bronze is a good alternative for the bearing material. make sure you add an oiling point in the top of the drop link and through the crosshead to the bearing.
cheers,
julian