Is the bike down on performance or burning oil ? if not why fix what isn't broken. My bikes has been "ticking" ( confirmed by a Yamaha mechanic) for the last 80,000 miles, it doesn't need oil top up between changes (every 6,000 miles) and is not down on performance. I have had a new head ready to go on for the last 50,000 miles but its sitting in the spares box surplus to requirements at the moment. Bike is at 122,000 miles at the moment and still goes very well.