I'm assuming that this is the petrol 2.0 ltr?
so the 1st thing to check is are all the cylinder compression's within 10-15 psi of each other ? as this will show if the engine is basically healthy and if the cam timing is correct
It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine and so then be low on power
and I'd also pull the intake hose off and with the ignition on but engine off and have your assistant press and hold down the accelerator so you can check that the throttle blade is opening fully
Ok the advice about checking compression still stands and its also worth checking the fuel pressure too as diesels are very sensitive to this