Keeping the SP cool is a bit of an art form.

The exit ratio is from 2 to 3 times the inlet, and the SP's run hot, so you had better calculate on 3.

You might have to build in a funnel for the motor, so that ALL the incoming air is forced down over the motor.
Just consider that any path past the motor is the path the air will take in preference to going over the motor, so your ducting has to give the air no option but to flow over the motor with no way to go straight through to the exit.
Removing the dummy engine might actually make things worse.

On your exit, consider adding small lip to the front edge as the high pressure flow over the cowl could be causing a blockage of the exiting air.
The lip will cause a low pressure trough which will aid in extracting the hot air from the cowl.

I had a 120SP in a TF Corsair with these problems, and currently have a 60SP in a scratch built Sea Fury with a funnel duct that works. The bottom half of the front is blocked by a baffle, the air flowing into a funnel shape tightly fitted around the motor with the lower section open. Took 3 tries to get the baffle shape working but now it's fine.