Righto Norm,
I had a look, and must have discarded the old needle/seat [usual practice], when I put a kit through my daily mower's carby.
But I did find the fitting instructions, in the OEM Briggs parts kit box that contains the leftover bits.
That says to use a 3/16" pin punch as a seat installation tool, and as I recall, that was not a great deal smaller in diameter than the seal- say 1/64" or so. So that's the ballpark you're looking at.
To drill out the Victa end cap, a straight flute drill, or slotting drill/end mill with the sharp corner stoned off the cutting edges, would be the go I'd think. That's if you end up making this a standard ops procedure, of course.
