Hi Timmy,
When desirable tubes become rare enough, solid state replacements do begin to appear on the market, in this link you will find solid state replacements for several of the one volt tubes used in portable radios like the Zenith T/O. It started with the somewhat valuable 1L6 and 50A1, and now even more common types like the 1U4, 1U5, 3V4, and others are available.
Not my website, I have no affiliation with these tubes for sale whatsoever, I enclose this link only to show Timmy that what he is asking is already a reality in the old radio world......
https://picclick.com/Complete-Set-So...891821945.html