When it comes to restored Japanese birds, from the little I have seen on the web, I have found the groups that handle them to be quite professional. As Shep854 said, its expensive. Japanese have restored a number of things to sitting condition but the only thing I can think of that can actually move that they restored was a Type 89 tank. Although I have seen some comments saying things like "they restore too much", they being not just US but also Russia and such. Also flying without the correct engine seems to turn down the opinions by some here. So some may be bothered a little if a G4M is flying with modern engines installed as that would fail to preserve it as what it was. Well could be a bit nitpicky. Probably nothing wrong with having both worlds, some restored as best as possible to what it literally was, and have some restored to flying (or driving) condition if is sacrificing a little of the originality to gain a chance for others to experience seeing it flying as opposed to just sitting. But sometimes I think having them in a flying condition risk them being taken too much as toy things or stopping short at just national pride feeling without careful thinking about the fuller context of them. But yet having them flying could be a better at getting attention and thus more able to get people by their own interests on a path towards better understanding of the fuller context.
Just don't feature the G4Ms in a PH 2001 kind of movie