From the supplied hints, it wasn't fusion. If cold fusion could be made to work, it could be made that small, but it wouldn't be that powerful.
Think of a design that involves skirting the edge of excursion. A little whiplash from the dragon's tail.
I don't really recall if I've ever come across puzzle pieces that hinted whether it was fusion or more conventional nuclear processes. I wouldn't be that surprised either way.
My impression is that the magnetic technologies the ET craft afforded us--whether reverse engineered or via directly taught/given technologies--that the magnetic technologies could readily be used to contain all manner of things and forces. Perhaps fusion it is. I just don't know at all.
I have never been able to resolve via any individual or collection of puzzle pieces the seeming contradiction between deployed nuclear engines vs zero-point/free energy engines using energies inherent in the quantum levels of reality.
I'm pretty well convinced that the latter are true in 2-3 DIFFERENT ways. I accept, having watched Stanton Friedman's presentation in March 06, that it is highly likely that the rumors of our having UFO type craft with nuclear engines in them--that that's also true. Why both is beyond even my puzzle piece collection and very risky extrapolations or even guided flights of fancy.