Ok, I don't own a dirt bike either, but I sure know a lot about them, and your looking into a 125? A yz or what? If you are going for a YZ125, that would work for you, but you are better off getting a 2 stroke instead of a 4 stroke, and heres why, the 2 stoke, if you blow the engine, all you have to really replace is the pistons, where if you buy a 4 stroke, you have to replace the whole entire engine, and that cost A LOT.
Well I hope I helped you out a little.
L8erz.
Rach
P.S. If you have anymore questions, you can PM and I will try and help with what you need. :)