Back in the 1980s, I read a story in (I think) an American children's magazine; it was probably one of my brothers' magazines, aimed at elementary-aged students. I was hoping to find it for my own kids, but I have no idea what periodical it appeared in.
It was only a few pages long, and was called something like "My Cat Was a Cyborg" (or maybe it wasn't "My Cat" but the actual name of the narrator's pet, which I now have a vague memory may have been "Slugger"). The young protagonist finds an alien box that offer instructions for how to build all sorts of alien technology. The first thing it suggested was a cyborg, and the instructions called for a certain amount of "inert protoplasm" to be placed in the open box. The kid places his sleeping cat inside, and it is transformed into a half-cat, half-robot cyborg.
Eventually, the protagonist comes to miss his normal cat, but he can't figure out how to get it back. He does something, and the aliens who abandoned the box eventually come back to reclaim it and return the cat to normal.
I am pretty sure that this appeared in the same magazine as the story I asked about in this question: Seeking a children's story about living in a horrible "intelligent" house. I have a feeling that, at the time, the magazine seemed to be cheaply produced--thinner and less skillfully illustrated than 3-2-1 Contact or Ranger Rick. Whatever it was, I suspect that the back issues are not to be found anywhere out there on the Internet. I think another (non-SF) story from the magazine may have been called "Operation Lima Bean," about a kid who avoids eating the titular legume by giving them to his dog; but rather than eat them itself, the dog (named "Shep"?) hides the beans in the boys baseball cleats.