Re: Digital FM radio tuner.
I don't think you can replace with another cct and expect to integrate it into the same controls as the rest of the audio system easily,
especially if it is all on the same board.
It would be more of a challenge than to just pay someone to fix the existing tuner.
If you wanted to go down the road of building your own digital tuner, then it is no different than an analog tuner, except that you can use a thing called a varactor diode
to adjust the capacitance, via a DAC. But the other thing you will probably need (different to analog tuners) is a PLL, to ensure the frequency
is correct and stable.