Change your oil and note the color and if any water is present. concerning your plugs there is no need to change them and a hotter plug if available might promote detonation at full throttle and that could cause permanent damage. If your plugs are in good shape with sharp edges just clean them with carb cleaner and use air to blow out any debris.
The dark color of the plugs is an indication that the engine fuel mixture may be rich and stalling at low speed would back that up.
The best thing you could do when this 4 stroke is broken in would be to use Royal Purple synthetic oil it's superior in every way for this engine with better protection, performance ,fuel economy, and the internals will be so much cleaner.