1. I don't understand the question. Are you asking how many hours are currently on the motor or how many hours can you have before experiencing serious problems? You can develop a problem at any time period, but it depends mostly on how you maintain the bike.
My motor has approximately 75-85 hours on the motor, and my bike is a 2003 model.
2a. My motor is stock. I have a Full Yoshimura exhaust and the carbs are re-jetted.
2b. The right time to change your engine oil per Yamaha's guidelines is every 40 hours of use. Depending on riding conditions, (dusty or wet conditions) 20-25 hours of use. It's honestly a preference call with most owners. I choose to change it after every 15-20 hours of use.
3. The right time to change the piston would be indicated by a drop in power, or as Aaron said a compression test on the cylinder head.
4. Depends on who I'm with. My riding style is agressive, with several rest periods if I'm with a group. If I'm alone I don't get stupid just incase of an accident.
~Brian