A 2wd Big Bear 400 should top 50 easily. The 4x4 ones are a little slower, about 45 or so. The gearing and weight add up to a great workhorse but not one you'd like to take to the Kentucky Derby. Sell it and buy something with a bigger engine like a Grizzly 660 if you want to go fast. As for the post of a Big Bear punched out to 725cc, I don't think that's even possible unless someone managed to make a Grizzly 660 engine fit in a Big Bear.