Hi, My Nephew has a DNA 50 with exactly the same problem. I cleaned out all the carb, jets etc along with the air filter but all seemed ok.
I popped in a new plug which improved things a little.
We did find a split in the rubber inlet manifold where it clamps to the carb. This would suck in additional air giving a lean mix and poor running. The split wasn't obvious at first but worth checking.
Regards
Dave
July 2008