So... is Magma Taishi finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: June 21, 1993
Manga finished: August 01, 1967
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
An alien force named Goa threatens Earth with invasion, demonstrating its power to a family, the Murakamis.
Refusing to surrender, the Murakamis find an ally in Magma, a golden giant created by the wizard Earth.
Along with his wife Mol, Magma is tasked with defending the planet from Goa's monstrous attacks.
A young boy, Mamoru Murakami, is entrusted with a special whistle to summon Magma and Mol in times of need, forming a strong bond between them.
Earth also creates a duplicate of Mamoru, named Gam, to further aid in the fight against Goa's forces, ensuring Earth's defense.