Stick with the Sigma it's a great engine - tune it to get the most out of it - plenty of parts available.

Whilst the gearbox should work on the Duratec family of engines but why would you swap it... you might as well change model as suggested.

Engine transplants only really work when they are cost effective in comparison to model change - which at your age of car I can't imagine it is.

In your case either a late model +8 or Roadster both would easily cost in - in comparison to the headache of forging a new path to an engine upgrade.

If you really like the 4/4 style then possibly the 4/4 Aniversary edition might tick all the boxes.

I think for you there are two choices:
1) Tune the Sigma engine you have 200bhp is achieveable and zingy fun
2) Ride the torque and get an older v6 or +8

[Linked Image]