RE: MVVS 45 IRS/MG - spark plug getting loose
Balance the prop first. Track and mass balance. The 43cc engine has very low vibration levels, so any excess vibration is from prop unbalance.
The included tool in it's original form was for centering the plug only, and has a soft steel bar, which is too small to proper torque the plug. Replace with a slightly thicker (6mm or 1/4" ) and longer piano wire bar. A standard socket or pipe wrenchwill need to be turned down in Outer Diameter, but that would another solution allowing for stable rod inserts.
A properly tightened plug will not loosen.
Added in edit:
Proper tightening for conical seat plugs without washer is 1/16th of a turn after the plug seats well.
I tighten mine about 1/12 th, because the plug boot is clamped between plug seat and plug taper, so it adds another element, and 1/12th is easy to guess with the hexagon flats of the wrench.
I took the original soft bar out of the wrench, and insert a philips screw driver to use the original factory tool.