Ok, first off, an MBK Nitro or Yamaha Aerox as they're more commonly known, doesn't need a battery to run. Very few 50cc two strokes do. I tinkered with tons of these things for years so I can safely say I know what I'm talking about.
Remove the soark plug first of all and inspect it. If it looks old and knackered, replace it. Then whilst it's out of the head, put the cap on it and ground it on something metal nearby, must be bare metal. Hit the starter or crank the kick wth your hand slowly. It'll be easy to crank as the plug will be out so you'll have no compression.
BE CAREFUL. DO NOT TOUCH THE PLUG ITSELF OR YOU WILL SHOCK YOUSELF. ONLY HOLD THE PLUG CAP.
You need a clean blue spark. If it sparks to one side or tje spark is in any way orange, replace the plug. NGK do some good two stroke plugs for a fiver or so.
If the cap doesn't fit nice and tight, replace it. You can simply unscrew it from the cable and screw a new one on.
If it sparks nicely, the ignition is fine in which case it could be a magneto coil, but try that first to eliminate it and report back.
I got 22bhp out of an Aerox once. Mental didn't quite cover it
