Several decades ago these replacements were for sale. They even had the pinout of the tubes for direct replacement. I do not think that there is a market for them now.
About 40 years ago, I had a tube type car radio in a 1961 Buick that I replaced tubes with JFET's. The tubes operated with 12vdc plate voltage making it real easy. Audio output was a Delco PNP 'doorknob' packaged class 'A' power amp on a big heatsink. After retuning for slight difference in stray capacitance it worked fine. 'Wow' instant on, no more waiting for tubes to warm up.
You need depletion mode FET's to emulate a vacuum tube.
Valve heptode mixers are noisy and suffer from chronic intermodulation, replacing them with FETs would improve any old receiver. I intend to this as a project, but the draw back is that IO bases are hard to find!!
Frank