If the loco has an older pancake motor, it may either need a new motor shield (if the shield is completely made of metal), or it may be possible to break some of the conductors to make it suitable for a decoder. A German forum post I found suggests that the 4305 is of the latter type.
The dark red arrow to the left indicates a contact that needs to be severed. The green arrows up top show RF capacitors that should be removed. All the other arrows show the colour of the decoder wire that needs to be soldered there.
You can get the 430073 with dcc sound factory fitted, so it must be possible.
A few years back I had both my Fleischmann 4033's converted to DCC sound, and although they run OK, I felt they never ran as good as when they were DC only
Look, I'm no expert with these motors. But that looks like it's metal without any conductors on it. In the overview from Pfeiffer it looks like it's 504702. So you may need to replace it. Part number 504730 may a suitable replacement part for this. You can still find those on eBay.
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