Gronk, I'm pretty sure a stock 650r weighs more than 290? that's probably the dry weight and I can't find my notes but I thought it was 306 ish with all fluids and no gas. Everything contributes to the weight loss,.. lighter wheels, brakes, shock, swingarm, lighter headlight, etc,.. The missing airbox saves nearly 5 lbs and the missing shock linkage was another couple lbs. The better suspension makes a huge difference too, no swapping in the whoops and no shock fade.
Byron 555, the Freeride is not really very light and it has less suspension travel. There's another one called the Bergham R250 that is the same way. Hardly any lighter and less travel. They both have all the complication I don't want on the trail with fuel injection, e-start and water cooling. however they both validate the concept that less power is better if its the right kind of power. My little Powroll 218 stroker with torque cam is the friendliest, smoothest most ridable power I ever experienced. That was the biggest shocker, coming off a 650 I thought the 200 would be pretty lame but it will pull 6th gear in deep sand and hang right with a 450 in michigan trails. I may be using 80% of my power all the time but it still has more than I need! The 218 has about 30% more torque than stock and that feels like a lot more when you shed so much weight.
Building around the little XR motor means I can swap in many of the chinese clones as you pointed out but also the CRf230 engine with its e-start and available hop up parts. These old school engines may seem uninspiring but they respond well to hop ups and really wake up when installed in a radically lightened bike.