Here is a speculation on a freewill system. Just some aspect of it.
Even if the there is a physics engine system in the brain. There is a faster way to generate actions than to simulate them in a would be 3D world.
That is to generate a brain action sentence. Like most actions I can think of. They can be described in a book. In single sentence of words or more. Then one way to generate the next action is to predict it by the input of the previous action sentences. This would be the brain action sentence system.
The procedure of predicting a plausible action sentences should be quite fast. Like generating a manuscript for a personal biography movie.
Another system would be the will prediction system. If you describe the will in a sequence of different wills. You could similarly predict the next will.
If the action sequence with the new predicted value is transformed to a feeling. Or some other common format. Then it could be compared with a similar transformation of the will sequence with the new predicted will.
The idea is then that if the feelings from all the transformed systems are equal. They coincide. Then the will is set to the new will and the action is performed.
If they are not equal. The brain sentence prediction is regenerated with the error as the latest input. Because the will prediction is probably less likely to be wrong.
So basically the idea is that all systems in the brain predicts what they are going to be next. Then the idea is that these predictions are transformed to the same format and compared. If the comparison is successful the action is performed.