For Athearn Genesis F45/FP45 locomotives, the UFIT-27D9-RC1 and UFIT-27D2-RC1 Rectify systems drop right in. Choose one speaker in either the front or rear location or load both spots for maximum power and output.
 
For the F45, both the 9mmH 27D9 and 12mmH 27D2 systems fit either or both locations in every instance I have seen or heard about.
 
The FP45 requires the 9mmH 27D9 in the rear location to clear the radiator shroud. Some FP45 front-speaker locations have room for the 12mmH 27D2 system, while some need the 9mmH 27D9. You'll need to measure your particular locomotive.
 
PCB-style decoders such as Loksound Direct and Tsunami2 PNP fit perfectly. TCS and Decoder Buddy motherboard installs also fit perfectly. The long tabs of the stock PCB do interfere with the front speaker a bit, though using the 9mmH 27D9 allows the PCB to sit over the speaker without interfering with shell placement. The stock PCB is discarded in most cases anyway.
 
The speakers come fully assembled and tested.