Cube purists will likely hate this, but
here's another possibility [ATTN MODS: not my auction]. That link is for an iMac G3 USB keyboard, specifically a graphite one that will at least match the look of your Cube. It's important to note that the original iMac G3 keyboards are the only ones that have an integrated power button.
Back in the day when Apple first began incorporating USB into their Macs, they began by including a power on/off button on the original iMac keyboards. However, the USB consortium had a problem with Apple using USB that way, since it was a nonstandard application not supported by their protocols - or so it's been said. Anyway, Apple dropped the power button from their USB keyboards going forward, and haven't returned to it.
But I can tell you from experience, that the power button on the iMac G3's USB keyboard performs just fine on every machine I've connected it to. In the past I've had one hooked up to a Sawtooth Power Mac G4 that was a bit inaccessible underneath the desk, and the power button on the keyboard started up the Sawtooth regularly without issue. If you can't repair/replace your Cube's actual power button for what you would consider a reasonable cost in time/effort/dollars, you might try picking up one of those old iMac keyboards. They vary widely in condition, however, so do your diligence to make sure you're getting one that looks decent.