Maybe even nuclear engines before fusion. The Botany Bay was powered by them anyway. (Hmm I never thought of this before: No DY-100 class or DY-500 class in the opening montage for ENT. Evidence of a parallel universe?
)
Agree with the idea of the separation of power generation and warp field production. Allows for hypothetical ship A which uses fusion for both Newtonian and non-Newtonian propulsion and able to switch between them because there is linkage to both systems. (A M/AM reactor could also be included but is not required.)
Hypothetical ship B uses fusion solely for Newtonian propulsion. No linkage is included to the non-Newtonian systems because the power output of fusion is no longer sufficient for those components and they are powered by M/AM instead.
Other combinations are also possible...