Howdy. dobry dzień. You say it is a 78 KZ650-C2. That is probably correct. But the date of mfg isn't necessarily the model year. So it could be a 79 650-C3. You can check the frame & engine numbers at CycleChaos.com. A 650C is a standard model, which is a good thing. The seat is not stock. The stock seat is flat.
Airbox. I think you can use the airbox from any 77-80 KZ650 (KZ650-B/C/D/E/F). If the airbox ducts are hard, removing the carburetors can be difficult. If hard, I would get new ducts.
Carburetors. I would replace the float bowl gaskets & o-rings. A metric nitrile o-ring kit should have some sizes that will fit. Or get a metric viton o-ring kit. Viton rubber is better than nitrile rubber. Sometimes you can rejuvenate paper-type gaskets w silicone spray. saturate them, place in sealed container & let them sit overnight. If you get new gaskets, look at yours first & compare. There are 2 or 3 different KZ650 float bowl gaskets, so get the correct gaskets.
Front brake master cylinder. You can use the Fr brake mc from just about any model with dual disk Fr brake. Even from a modern motorcycle.
Criteria: 7/8" handlebar mount. mount angle (most have a level mount & others have an angled mount). dual disk (5/8" or 16mm piston bore). mirror orifice (some don't have a mirror hole). With a modern mc, you may have to splice the brake switch wires. easy.
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