If it's a factory Mitsu turbo, the wastegate flap will already be there.
The large bulb (vacuum diaphram) with the arm protruding from it (or as it's called, an actuator) leads to a small metal arm on the external of the turbo, which leads to a small flap on the exhaust side of the turbo housing, this is the wastegate flap.
You can 'if you were chasing numbers' get these welded up, and an external wastegate unit added off the exhaust manifold. But again, 'only if chasing numbers'
Re: a rebuild, it's always advised to do it, my turbo I added to my Magna from a 70,000k rvr felt perfectly fine, but it is blowing a bit of oil, despite the shaft feeling solid.
You can either DIY, which personally, even I wouldn't be game to do, and I am pretty hands on.
Or take it to a workshop specialist.
