If spark plugs look as though it's running too hot then it is probably running too weak (or, less likely, has a bit too much advance). Free flowing pipes and a free-er flowing air filter may make it run even weaker. Suggest trying a richer main jet in the carb. If you drill one of the downpipes and weld on a threaded boss you can add an 02 sensor and read the signal from that at various speeds and loads to ensure correct stoechiometric burn. There are various kits available that include the boss, the 02 sensor and the reader.
