jjeff is correct, no analog tuning of any kind on the new Magnavox units.
You can solve this problem cheaply by connecting a compact VCR between the DTA and your new Magnavox. The wiring would be DTA>VCR, with VCR tuned to the DTA output channel, and line outputs of the VCR connected to line inputs of your new Magnavox. Leave the VCR powered on permanently, and it will function as bridge between DTA and new Magnavox.
The recording quality will be slightly worse than what you get with DTA connected directly to the tuner of your old Magnavox, but these DTAs aren't great to begin with anyway: if you were happy before, you'll probably be happy with the VCR crutch as well.
Unfortunately, there are few (if any) "reverse RF modulators" available to the public as tiny accessories. One can buy a very small cheap box that converts line outputs to channel 3/4, but going the other direction (ch 3/4 to line outputs) is generally best done with a VCR. A small VCR is easily found for $20 or less on Craigs List, or just ask around: odds are, someone you know has an old VCR they don't need.