I'd probably stick with a stock motor 125, with the exception of a flywheel weight if/when you desire. Bike armor would be very wise; bark busters, a skid with good coverage, radiator guards/braces.
It's really not 'too' much about the bikes power out there. Its embarrassing to say, but funny none the less; I let a guy around me in one HS race - he was using a bicycle bell to alert riders ahead of him that he wanted to pass. "bring-bring, bring-bring". I literally laughed out loud at that - I let him around, and saw that he was riding a very stock looking CRF230. "WTH", I thought. I chased him, but he was gone a few turns later. I now respect the underdogs and 'little' bikes out there.