
Monster Island was ADV, ADV became Sentai, but there's almost no chance that Sentai would have anything from those days considering they had to dissolve before becoming a new company.
I think the only chance to get the recordings in their truest forms would be one of two methods:
- Getting a japanese-speaker to get in contact with the original composer
- Trying to recompile the music by taking the english and japanese audio tracks and trying to transpose them and remove the dialogue.
I'd love to see it happen, but I'm not optimistic.
edit:
IMDB lists the musical staff as:
Junji Fujita - Music Production
Riyu Konaka - Vocalist/Lyracist (Sang Look-A-Like)
Mitsuhiro Tada - Arranger


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