Will a yz 250 motor fit ablaster?

No, a Yamaha YZ250 motor will not fit directly into a Yamaha Blaster. The two bikes have different frame designs, mounting points, and dimensions, which means significant modifications would be needed to make the engine fit. Additionally, the YZ250 is a two-stroke Motocross bike, while the Blaster is a two-stroke ATV, so their setups and performance characteristics differ. If you're considering such a swap, it would require extensive engineering and custom fabrication.