The first part to replace is the stock plug cap unless your kit has the newer soft molded cap. after that,Coil, inner tubes, cables, brake pads, fuel, oil, chain (recent kit chains are better but still a crap shoot), seals, gaskets (head, base and exhaust), fuel filter, plugs, fuel line (Modern gas sucks), Bucking bar and ball. In no particular order.
My most often seen problem is probably electrical. Bad coils 99% of the time. Any serious rider should have a spare. But the recent kits have better coils as well so hopefully this issue is improving.Remember to SEAL THE COVER even if you have no plans to ride in the rain. All it takes is a puddle...
A lot depends on how you ride too.
A spare top end is not a bad idea is you plan to do a lot of mileage.