I believe it depends upon which turbo you have, but I would have to look at them to make sure. The mitsubishi style turbo I believe does have a removeable housing that has your headpipe mount studs in it. If yours is of this style then yes you can remove the nuts that mount it to the back of the turbo and fix your busted stud right on the bench. A word of warning though, I have done a few of the studs you are talking about and often times one or of the nuts that you have to remove from the turbo to free up that housing are seized as well and will jack up the studs that they are mounted on if not break them off too. So, use some good penetrant and if you have the time allow it to soak in overnight if not over a few days time. If it does seize then take your time and work the nut on and off, on and off until you can get it to come off. Once off you can drill out the stud, if you are not set up to do this I recommend that you seek the services of someone who is. Where I am at there is a place called Jerry's broken drill and tap removal and if you brought it in to him he would probably charge you somewhere in the area of $30.00 bucks or so to straighten it out. If you are like most people you probably do not want to spend the money on it but trust me if you are not set up for this kind of work then it is money well spent. If the stud was sheared off because of the weight of the headpipe then you have a decent chance of drilling a small hole in it and having it unscrew right out. If it is seized in there and it was twisted off because it would not unscrew you may really want to have a pro take care of it.

Replacing the turbo charger prophalactically is something that you can do but it will not be cheap. The labor involved is not minor but it will pale in comparrison to the parts price on the turbo. I have not done any 900 series turbos in years so my pricing may be dated on it but they used to go for $1200.00 or so just for the turbo. Add gaskets, oil, filters, coolant, labor, and etc to it and it gets spendy pretty quick.

Finally, back to why the stud is busted, if you are missing the bracket that goes around the head pipe and bolts to the backside of the bell housing bolt you might want to replace that. That bracket often times cracks from engine vibration and is removed. This bracket is there to cut down on vibration and to support the headpipe so it does not saw through that lower stud.

Good luck,
Mark