Hi!
I don't agree about putting " the- largest-engine-that -fits -in the cowl" idea! Some newbies that have only flown 5-10years maybe think so but this is not the way of doing it if you want a good flying (scale) model.
The best way of doing it is to first build extremely light and then put in as light engines as possible.
I use two OS FS.26 four strokes in my DC-3 . Weight is 3,6kg...flies like 2m span sailplane.