XEMX makes two separate conversion kits (the air-cooled XE2 and liquid-cooled XE4) for Yamaha's YZ250F and YZ450F dirt bikes, but the brand is currently only accepting pre-orders for the XE4 model.

If you've ever attended a Motocross race and heard the throaty growl of powerful four-stroke dirt bikes as the riders navigate the twists, turns, and jumps, it's hard to fathom a world where that ...