Interestingly, while TechManual states that none of the permitted engines take up internal space (at least as far as the ability to mount weapons in any arc goes), the Advanced Engine Master Table in TacOps (p. 308) does have an explicit "Vehicle/Fighter" column for this purpose, of which the "Fighter" portion of the header (there's only one number in each row below it, which would presumably apply to both vees and fighters) has to my knowledge not yet been errataed away...and it explicitly lists compact, light, and XL engines as well as the more TacOps-related ones.
So it currently looks like an Inner Sphere XL engine takes up no slots in a fighter under standard construction rules, but two (either in the rear or split between the wings, the wording of the footnote below the table is unclear) as soon as TacOps comes into play... ???
(No extra slots for compact engines either way, though. Sorry.)