How many hours does it have on it? I'm assuming that when you said fuel got into the oil sump that means that the oil level got so high that the engine was sucking engine oil back in and was dieseling on that. If that's the case, the oil might have gotten really diluted, and a number of things could have happened. The turbo bearing could have gotten galled, so you should check that out - a bad turbo bearing will keep the turbo from fully spinning up and it will eventually die altogether. If the oil got high enough, you could possibly have blown out some rings - so doing a compression check on each cylinder would be a good thing to do. Most iimportantly, you might want to do an oil analysis to see if you have any bearing metals in there - if the oil got diluted enough, the bearings could be on their way out and if the main bearings are galled, you'll definitely notice a loss of power before things get worse.
If you have to do a bunch of work on it (worst case scenario), you might want to consider upgrading the engine parts to the TN75 equipment. I can't remember offhand, but I think the only difference between a TN70 and a TN75 engine is the turbo. The TN75 uses a turbo that provides more boost, and you get more horsepower. If the turbo is bad, I don't think there's much difference in price between the TN75 turbo and the standard TN70 unit. I could be totally wrong about this, and I'm sure people will chime in with corrections.
