A friend wants me to do a complete rebuild (top and bottom) on his YZ. His cases are broken (chain derailment) so that needs opening for sure. What should I do/look for in the top end? He bought the bike used and from the looks of the bike it has lead a pretty hard life. Should I automatically replace things such as the timing chain? What 4 stroke specific tools do I need? Sorry in advance for the stupid questions. None of my bikes have valves so I need some educating here. Thanks!