Thank you John. It will be much easier then to just run the engine without the shaft belt but I did read I think on the 145 Forum that the shafts would need to be somehow fixed into a position to stop them from rotating randomly and thereby causing dangerous vibrations?
The balance shafts work by creating their own vibrations which are in opposing phase to the 'natural' vibrations created by the reciprocation and rotation of the pistons, rods and crankshaft (i.e. the relationship between crank rotation, piston reciprocation, and the combined rotation and reciprocation of the rods, the geometric relationship of which isn't as straightforward as one might think). For each vibration originating from the crank / rods / pistons there is a counter vibration originating from the two balance shafts (rotating at twice crank speed), the net result being that there is less apparent vibration of the engine as a whole, so less vibration to be passed into the chassis.
The 'balance' shafts do this because they are carefully designed to be inherently 'out of balance' themselves (to a very specific degree, achieved with weights / masses that are offset from the axis of the shafts). If the belt is removed (and thus not being driven by the crank), then due to gravity each balance shaft will slowly rotate until the heavier 'side' of the shaft (created by the offset weights / masses) is sitting at the lowest point of rotation, and if you were to turn the shaft to any other position it would again rotate back to the position where the heaviest side of the shaft is closest to the centre of the Earth.
In use, with other inherent engine vibrations occurring ('exciting' the balance shafts), the beltless balance shafts (rotationally unconstained) may 'swing' around the shaft axis to some degree, somewhat like a pendulum being shaken. But they won't ever rotate fast enough to cause the vibration they are designed to create. They might 'swing' but relatively slowly, and probably wouldn't even swing a whole single rotation (but even if they did, it would be too slow to matter).
Note that the 'unbalanced' forces which create the inherent 'secondary' vibrations do not disappear, they are merely 'masked' by the counter vibrations created by the balance shafts. Also note that 'balancing' the engine by balancing the rotating assembly (crank / flywheel / clutch etc), matching the rods (so that all the little ends weigh the same and all the big ends weight the same), and weight matching the pistons, does not reduce the inherent 'secondary' vibrations that exist in a recipocating piston engine. No amount of 'balancing' affects these secondary vibrations (or the forces that create them), they are inherent in how a piston engine works.
However, 'balancing' an engine will reduce 'primary' vibration, assuming that primary vibration was a bit off to start with. Modern engines (out of the box) are generally much better in this respect than in the bad old days, i.e., with a modern engine (made with modern technologies) primary balance is unlikely to be significantly 'off' compared to years ago, so typically there is a lot less improvement to be made than there once was.