You can do it with a couple transistors, absolutely; find the last surviving, oh say BFR92 or better, put one or two together in the boring old FM microphone / tape player to radio adapter circuit, tune it to 2.4x GHz instead, and there you go.
Won't be at all stable, lord knows power output, purity, etc. All that is left as an exercise to the reader
Actually, need not even be too awful if it's done with a dielectric puck, transmission line or cavity resonator.
But ready-to-go things, made responsibly with properly designed channels, limited bandwidth, well defined SNR, etc., yeah dunno; a few possibilities but most will be too narrow band or low bitrate (audio?) or too specialized (video?). Though there's a possibility a video converter is just that, a converter, and doesn't care about what composite or other signal is fed into it -- could be worth trying to feed one with your signal anyway, or maybe with some minor encoding if you can afford to (mimicking sync pulses to set DC offset, perhaps?), and see if you get the same thing out the other end.
Kind of the same principle as, there's at least one USB (or maybe it was Thunderbolt) to VGA converter, that's literally just a triple-channel DAC fed by a frame buffer and DMA; essentially 100% software defined / CPU based graphics adapter, how horrible right, but, I mean PCs can get away with that kind of thing. (Remember the days of "soft modems" dragging down the Win98 PC?
Needless to say, days
long gone now!) This had been noticed by some avid hackers, who repurposed the frame buffer into an SDR transmitter, just plug a strip of wire into the VGA jack and go.
Wouldn't have been possible if it was in any way VGA-purposed, i.e. internal frame buffer, sync generator, automatic blanking, etc..
Tim