I removed my old harness, stripped all the tape, replaced and damaged wires, added some new circuits , and repaired/replaced terminations as needed and re taped it. Made sure I marked / kept track of where the circuits entered / exited the main harness.
This depends on the condition of your existing harness. Could just use it for a model if in very poor shape.
For parts britishwiring.com has all wire, fuse boxes, fittings and tape.
This site also has a reference for recommended wire sizes for all the devices being fed.
THING 1 - 1973 88 SIII - SOLD
THING 2 -1974 88 SIII Daily Driver - SOLD
THING 3 - 1969 88 SIIA Bugeye Project
THING 4 - 1971 109 SIIA ExMod - SOLD
THING 5 - 1958 109 PU
THING 6 - 1954 86" HT