Mechanic in a Bottle will definitely help the engine to start provided it is a fuel related problem.
There are other possible reasons that an engine will not start. If there is no spark at the plug (ignition system), the engine will not start. There could be an internal engine issue (lack of compression) that is causing the engine not to start. It could also be a muffler problem. If the muffler is plugged, the engine will not start. Insects can build nests inside the muffler that will plug it up.
If Mechanic in a Bottle does not solve the problem, the unit will need to brought into a service center for repair.