Most likely it is still copyrighted. The country of origin is Japan. Ordinarily foreign films are protected for 95 years from date of publication. That varies from country to country, with Japan being one that offers copyright protection of as few as 50 years. However, even if it were only a 50 year copyright, it will be protected until sometime in 2027.
It is also known as: Kyokutei Tankensen Pora-Bora
The production company is Tsuburaya.
There's a good book about Copyright/Public Domain info that I picked up and would recommend to anyone interested in researching this kind of stuff, I've provided a link below.
The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More by Attorney Stephen Fishman
You also might want to look at another of his books:
The Copyright Handbook: How to Protect & Use Written Words
That's the most info I can give at the moment.. I highly recommend that first book.
The short answer: it's probably still under copyright, but you could always ask (nicely) for permission to do a remake, who knows, maybe they'll grant you permission for cheap, or free.