I really like it.
The entire concept is really good. I love the way it manages to tell a unique story without relying on the introduction of a bunch of new in-universe concepts and plot conveniences; the Makuta’s motivations are realistic, and Zance’s experiments are all based around existing Bionicle technology.
The neat thing is that we already know full revival of Matoran Universe beings is possible using the technology on the Red Star. So, to add to your story, I would go on to specify that, during the briefly-successful revival of the corpse, Zance had accidentally recreated the method used to revive beings on the Red Star. This would explain why the dead Toa spoke Agori, as if Zance had activated some kind of “factory settings”.
Then, if we care about the exact science behind it (which I like to think about), I propose that the reason the reanimation failed so quickly is because the power source that Zance had cobbled together was unstable; stability is canonically known to be an issue with the power sources of both the Matoran Universe and the Red Star. The Great Beings stabilized it with Energized Protodermis, but Zance has yet to figure this part out.
There is, however, one potential canon issue with this backstory, and it’s a pretty big one: any Toa killed by the Makuta should have been revived on the Red Star, leaving Zance nothing to experiment on.
Admittedly, I’m not 100% sure on how the Red Star works. I know that it can’t revive anyone whose mind is too damaged, but does that simply mean that the reanimation procedures won’t work, or that the Star won’t bring them up at all in the first place?
The only way Zance could have any corpses to work on is if the latter were true, but, if we take that to be the case, then the minds should have been too damaged for even a partial revival.
However, the rest of the backstory is so good that I might be willing to introduce a bit of a plot convenience; perhaps, in addition to a bunch of unknowingly-unrevivable corpses, Zance lucked out and found a Toa who was damaged in just the right way to allow for revival, but to also prevent the Red Star from teleporting him up, and it was this Toa that Zance was able to temporarily revive.
Or, if we assume that the Red Star teleports beings up regardless of whether or not they can actually be revived, then Zance must have found a corpse with barely enough brain still attached to be revived, with the rest of the brain having been taken to the Red Star to be unsuccessfully revived (or maybe successfully; that has cool story potential in and of itself).
This story also has a lot of potential in the future; Zance could eventually figure out how to stabilize his machines and successfully revive the partial-brained Toa with the rest of his body rebuilt as well, probably cobbled together from various loose body parts that Zance took when recovering corpses.
And what about on Spherus Magna? Will he find a Great Being lab with even more technology? Or, since his machines are powered by the same thing that powered the Matoran Universe (albeit on a much smaller scale), will he find a way to keep the universal life support running around his island and stay in the Matoran Universe indefinitely with his revived assistant, unaware that the rest of the universe has died around him?
I personally prefer the second option, though maybe he’ll eventually wander outside long after the Battle of Bara Magna and find some Great Being tech.
Finally, there’s one other tiny change I’d make:
While Zance was originally designed before I knew about the concept of immoral Kanohi, it’s bugged me more recently that, in theory, no Toa would wear that mask. Intentionally wearing it to revive his own brothers, though, is a pretty good reason, so I’m going to say that Zance did not originally have a Tryna, and he only obtained and used it to begin his experiments. Maybe he found it in the abandoned Brotherhood lab?
EDIT: I also have an idea about Zance’s pre-Toa backstory: someone doesn’t just start doing this kind of stuff out of the blue, so I would propose that, even as a Matoran, Zance had some degree of interest in revival. I’m going to borrow a concept from @Willess12’s canon continuation stories and say that this interest was sparked when Zance heard rumours of Toa coming back from the dead:
After hearing this story (or a similar one), Zance had always wanted to figure out the secret, and the deaths of many of his fellow Toa finally pushed him to take steps past what Matoran society would consider acceptable.