Any engine you want. Place it where you want it on your craft.

Put another engine on top of your original engine. Nudge it until it takes up the same exact space as your original engine. You might want to do the same for the air intakes if it has any.

You have twice the engine power in one now, or more if you decided to add more than one, so make any other adjustments needed for the change in mass. Also you will use more fuel.