Well doesn’t the Japanese DVD have the complete track? The quality just isn’t very good but I assume they can use that track only for the scenes missing from the shorter version? I can of course send a copy of the disc if needed (or maybe you/they should get a cinemageddon account and download it from there ). I don’t think it exists in any better quality as far as home video goes.
If the original material is in fact in too poor shape, a better quality (than the one on DVD) track might exist on some Scandinavian print (The Danish Film Institute might for example have one, as they even have the complete English track for the Specialist). Then again the audio tracks on those prints might be in even worse shape and I assume Explosive won’t go hunting for a print from archives.