Yeah, I thought of that, but, with some work, I don't see why a recap wouldn't work (unless, of course, some unobtainable, important part has fried itself or something; if that were the case, I'd probably just get another IIsi PSU).If the recap doesn't work, there's always the hide the skinny lil' ATX PSU Hack! }
If I were to do that ATX conversion, I would be more inclined to do it to a IIcx/IIci/Q700, mainly because the PSU enclosure is more or less the same size as a standard ATX part, which would make it more or less* a drop-in replacement.
I just looked at it again, and the corrosion isn't too bad, actually. It's mostly on that IC.
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*Aside from wiring it up so that it interfaces with the logic board properly.