It's possible you might not need to do any mods to the radio itself - many radios these days have a '9600 out' at the mic jack (I seem to recall that some radios actually have a separate jack for this) to be used for TNCs. It would take some doing to see if that output is connected to the discriminator; if it is, you're half way home. MikeSome issues:
The digital modes that the Ft-897D can use out of the box are SSB AFSK modes.
You would have to modify the radio to have a discriminator output to receive FM based digital modes.
The frequency coverage of the receiver does not cover all public safety bands.
It could not scan trunked systems.
A dedicated scanner is almost always a better way to go to scan public safety.