No out-of-the-box replica watch is an exact match for its gen counterpart (if you are new, you should have read this). Mods generally include reshaping inaccuracies of the case, replacing rep parts with genuine parts (typically, crystals, crown/tubes, pushers (on chronographs), handsets & in more advanced cases, movements, cases or dials) or customizing to the owner's preference. In addition to the cost of the parts, most charge for the labor required.