Oh, sure, those engines, by themselves are free enough to be used.
However, reprogramming the existing game to use the features of the new engine properly, to fit within that engine's design paradigms, and to actually benefit from the change in engine is not a simple plug and play task. It would likely requires dedicated weeks or months of development time alone, since Toribash isn't simple. Furthermore, some of the parts of the game that actually have problems wouldn't see any benefit from this, since they're not dependent on these things. The cost of changing engine is primarily going to be measured in time and increased performance requirements, and Toribash doesn't (wouldn't) take advantage of many of the features these engines could provide anyways.