Hi, My dad has a -03 YZ450F (as do I) and he wants more power. He also does not want to lose any of the hit. We are considering everything including cams, larger displacement cylinders/pistons and exhaust systems. The new gases with high oxygen content are not an option, since they are not available around here. I would like to invite all experiences and suggestions. The top priority is horsepower/money and ease of installation. Some questions I have: Do big-bore kits require head-work if they increase the bore? How about the once that increase the stroke? Obviously the once that increase the stroke require the cases to be split in order to fit a new connecting rod? Does larger displacement require other changes to the intake or the exhaust? To pre-empt the inevitable: Yes, the bike is running perfectly. No, he doesn't necessarily want to be faster, just more fun. Yes, he can handle it. Thanks for all responses.