Yz125/yz144 advice

I just picked up an 06 yz125 last weekend. When I rode it, it had a ton of power. A lot of power for a 125. The owner didn't know much about the bike, they only had it for a few months but had a kid and didn't have time to ride it very much. They didn't know much about dirt bikes either. I didn't think much of it at the time, but after buying the bike and thinking about it, I think the bike has a big bore kit in it, which is exactly what I wanted to stay away from. The cylinder has "EDE 143" etched in the side. 143cc? And also, one side of the motor cases is old and stained, and the other side of the cases looks brand new. I've heard that for yz's you have to make clearance in the cases, so I have no clue what to think of the cases being different. I'll be buying a gasket kit and taking the cylinder off in the next week. Any advice on going from a 144 back to a 125?