Sure it’s possible – depending on your budget. 🙂

Not just a money thing, also depends how much effort your willing to put into researching how the things were made in the first place.

As “radio valve suppliers” returned 1.5 million hits, I don’t think it’s going to be that difficult to find people willing to sell you some of them. Almost certainly more than needed for one instrument.

Had never heard of this before your post, but that’s my opinion on your fad. Whether or not you can make it real, depends on you.

you can get valves easily especially european series, they are still widely used in audio equipment and also what remains of Cold War defence electronics infrastructure on both sides.

unlike the Theremin this instrument various versions still appear to be in production, the family business what made them still going with Monsieur Martenots son working with nerw manufacturers and copyrights/intellectual property closely guarded. I did find what was essentially a sales brochure in French for one but no schematics.

Someone in the USA has built a midi controller based around the technology and a Yamaha DX7 and shared the details (you will need to convert this into metric)

Dana Countryman’s Ondes Martenot Controller Project

but much of the sound of the original instrument would also come from the distinctive loudspeakers.

TBH I’d start with either building the controller and using it with newer digital kit or learning to build smaller valve oscillators/amps (beware of the HT voltage, make sure you have good blocking capacitors where needed)

dont u just need a casio keyboard, a wedding ring and a piece of string?