Let's talk about the same 13.5 lb. bike below.
Remove red crank, add s900 carbon crank with spiderless 34t, saves 60 grams.
remove inner chain guard. 10 grams
kmc 9 speed gold chain, cut down for single speed, 100 grams.
remove cassette, add crupi 14t, and GearClamp. saves 120 grams.
remove shifter/cables/housing, add s900 lever, saves 70 grams.
remove red derailleur, saves 147 grams.
saves another 1.25 lbs. now we are in the 12 lb. range.
Maybe find some r2 tubulars(260gram rims), rebuild them to some dati hubs, and drop another 1/2 lb. off it.