What you are describing is redoing the entire sound mix… There’s no easy way to separate the elements from the mono mix. At least not with the usual sound editing software. So it would be months of work. Also few people have a 5.1 system on their computer for sound mixing… I don’t 😃

Yet I think something can be done. I suggest doing it in steps.

First, go for a simpler Stereo mix, where you just add music from the stereo soundtrack recording.
There might be problems with this, since the mono mix already has the music in (phasing and time problems), but it’s worth a try.

Second, look at the Japanese version. It needs to be recut to match the US version I think, but if it’s usable (no sudden changes in music or sound effect it should be usable. The Blue Ray already has an English dub with surround sound…

I don’t mind giving this a look, PM me so we can discuss it a bit more.
No promises but I’d like to help 😃