The (SF354) PSU that I have is glued or welded shut, so it can only be opened destructively. I'd rather replace it if it was defective. Depending on which floppy drive there is in the unit, you might even not need the +12V. But if you do, there are a lot of +5V/+12V power supplies available (e.g. Meanwell GP25A series). You'll have to replace the connector, though.