wiring loom

I know obviously a new wiring loom is probably no longer available,and we all know about the matras poor electrics etc.i can read a wiring diagram to a certain extent but my knowledge of electrics is pretty basic when it comes to relays and diodes ,resistors etc.having said all that has anyone tried to replicate a wiring diagram ?? it surely has to be possible,if not a little time consuming.reason being that over the years owners tend to take short cuts(iv done it myself in the past) when it comes to wiring.i.e adding wires,replacing fuseboxes and not leaving a ref .etc