No wiring harness replacement needed but you'll need a few wiring terminals to hook up the CDI and rectifier/regulator. Also make sure the engine you get comes with carb, manifold, shift lever, starter lever, rectifier/regulator, CDI, etc.. You'll probably also need an air filter, fuel valve, and other odds and ends. On my change overs I did not use the engine mount gadget but instead drilled new holes. I also cut and rewelded the stock foot pegs. Made my own cables. Used the rear brake lever as a clutch lever (90s). Went with a no battery system. Cut and welded exhaust to fit. Etc.. Another thing you'll want is a 17 tooth 428 sprocket to fit the 20mm countershaft on the 125.

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