Mujitsu and Tairaku's Shakuhachi BBQ

I've been working on translating some notations that I have for shakuhachi into western staff notation for my girlfriend so that she can sing along. Now, I understand exactly diddly-squat about western notation. I wrote the notes down exactly as they apply for shakuhachi. Ro thus becomes D and so forth with the assumption that I'm working on 1.8 shakuhachi.

The problems I'm having have to do with the key of the pieces. Now, I don't even pretend to understand all the fine nuances involved here. However, most of the time the pieces I was working with use one of the few japanese scales. Often these don't seem very compatible with the idea of keys because no matter which one you pick you end up with something that isn't quite optimal. In most cases the least horrible choice was F minor. It produces plenty of notes that require natural signs but at the very least most flats are gone. Still, it feels wrong that although the entire piece may well be in Iwato scale the notation doesn't really make this obvious.

The question thus is, if you were to write down the notation for something like Edo no Komoriuta, what key signature would you use? Does it even matter? I'm working with the assumption that the piece has to go exactly as played on shakuhachi - that is, first note is U on 1.8. I'm sure most people would transpose the piece for western instruments but that sort of trickery is not possible here. I'm very much clueless about western notation and it may be that I'm just missing something obvious.

I would really appreciate it if someone could explain me the basic idea of key signatures in cases like this where the key isn't entirely obvious.

ps.

I used to wonder why people claim that shakuhachi notation is much easier to learn than western notation. I suppose that I know now.