it wont fit inside ha ha!
I assume you want to use the motor block from the eggliner to replace one in the FA?
there are 2 different mounting methods in the versions of the FA
read this to see how to adapt the bachmann motor block to your particular FA
https://elmassian.com/index.php/large-scale-train-main-page/ted-vignettes/ted-s-vignettes-pg-8/upgrading-aristo-craft-loco-motor-blocks-from-sleeve-to-ball-bearingGreg