Years ago there was a guy that was making a "solid state tube". for
sweep tube replacement. It consisted of two or three transistors an a
potted cast assembly on an octal base, very much like solid state
rectifier tube replacements. A customer claimed he'd busted one apart
and it looked like a horizontal output transistor on a plate, some
small transistors, a resistor and was put in black hard potting
compound with some smppth small rocks for packing. He was not
sufficiently on the ball to have made a schematic.

Failing that, converting the beasts to a transmitting tube of some
sort seems the only way to go,as sweep tubes are a thing of the past.
Apparently they take a lot of different internals from audio types,
which are the market today.