Hey
Did you fit a turbo to your nav engine, or did you fit a Terrano turbo motor to the nav?
First thing you need to do is fit a boost gauge.... this is vital. Ideally the engine will take around 5-7psi of boost stock.
I fitted a Terrano engine to my nav, and it was a vast improvements over the NA motor.
If your motor is not boosting correctly there is a few things you can play with.
Usually with a diesel it's the fuel mix, to lean and it won't boost or make power, to high and it will make bulk power but use lots of fuel and run really hot...to get into tuning you will need an EGT gauge..... minor adjustments can be made without, but you really need to check them with an EGT/Dyno.
The Na motor only has one fuel adjustment, so you will need to make it run richer in general to cope with the turbo. The terry has a on and off boost mix...so you can up the fuel on boost to get some power back.
The turbo actuator could also be old/leaking...usually opening to early, letting exhaust gas escape before the turbine can build boost... mine has an adjustable actuator rod.....if yours does - tighten it up a fair bit and see if there is any difference - but do with a boost gauge you don't really want to set the stock turbo over about 10-12Psi..
your exhaust can limit boost production to.....turbos LOVE free flowing exhausts.... if you using an old/original Nav exhaust it could be fairly blocked up. I removed the single muffler from my system, still using the navs small pipes - but the flow is more 'free flowing'...
Check your accelerator linkages is pulling the pump all the way to full throttle too.....
Blocked injectors will limit fuel and thus run lean and have less power...if you get to the ends of the list without any success you might want to clean/replace your injectors.
That should give you enough to try for now....but before you do anything id go get the boost gauge and get a baseline boost level...then work from there.