Engine Swap

Hi, a teen here looking for advice. As you may have guessed my wallet isn't too big and I'm looking to get a bigger bike.

I am fairly comfortable working on bikes, I go riding with my uncles and I'm looking to get something along the lines of a 125. My thinking was that I could possibly buy a frame for an 80cc bike and put a 125cc engine in it?

I know I would run into problems such as size but maybe a pit bike engine would fit considering there small size and Big output. Or on the other hand am I better off staying in the same make and for example buying say a kx80 frame and putting a kx125 engine into it? 

Any feedback is appreciated.