Dirt bike help for a 13 year old?

I am 13 years old and on the small side ( I'm 5'1" ) and I am looking for a 2 stroke dirt bike. I currently own and use a 2008 kawasaki klx 140L ( four stroke ), but I want something with a little bit more power and a powerband with some more "oomph". I was riding at a family friends track and was given the opportunity to use a ktm sx125 ( an adult bike. ) I rode it fine and felt comfortable on it but I had to "cheek it" a TON. The guy who owns the bike and the track recommended that I get a kx 100 but I will be growing pretty soon and don't want to grow out of it. My dilemma is should I get the kx 100 right now which fits me now, and grow out of it in a couple years when I hit my growth spurt or should I get a ktm 125 or other brand of 125 and have it be a little bit big but still rideable, and not grow out of it. I also want something my dad could maybe ride ( he currently has a kawasaki 250cc ) He is 6'3"
All help is appreciated.
Also I am mainly going to be riding on a motorcross track and a little bit of riding in the hilly wheat fields around our town. ( All private property )
 
I don't know man, most folks your height would not feel comfortable on a 125, but if you do then go for it. If buying another bike in a couple years is no problem then I'd start on something like the KX100 (personally I'd pick the KTM 105SX instead). Of course your 6'3" dad wouldn't be comfy on the 100/105.
 
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