Just finished installing a Digitrax SDN144K0a into an N-scale Kato F7A.  The engine frame was modified by Aztec Mfg so it has a machined slot in the rear to hold a round 16mm speaker from Tony's Train Exchange.  The decoder comes with front and rear LEDs, a capacitor, and a 13mm speaker.  Here are some pictures of the F7 disassembled and the machined opening for the speaker:

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I have finished the first install and just posted the results on my YouTube channel. 

Things that I learned so far: The LED has to be trimmed to be only 10mm from the back of the plastic to the end of the leads, the wire was the correct length to get the capacitor back behind the board and against the vertical where the speaker is located.  I used double-sided tape to hold the capacitor in place.  The leads from the 16mm speaker were the proper length to be soldered to the decoder (after removing the 13mm speaker). The only open issue is that I cannot re-install the engineer seats/cab back above the front of the decoder as the area is now too thick.  Because of that, the light from the LED fills the cab.  I will have to think about this a bit before finishing that part or I may just say good enough.  I think that the engine sounds pretty good.

UPDATED 30 July 7PM - Solved the extra LED light problem with a piece of electrical tape over the back of the LED. 

My YouTube channel is: https://www.youtube.com/user/tanzj