You'll need a turbo core, IPD sells them and there are plenty of decent rebuilders around. Or just get a used junkyard turbo. Either way, it's a good idea to replace the oil supply tube because they tend to coke up over time, resulting in oil starvation to the turbo (or bits of coke breaking loose & tearing the turbo bearings apart). If you're able to free yours up with some combination of tapping, turning, and incantations, I'd suggest running some AutoRX through to clean up the oil passages. Bear in mind it'll probably be a very short-term fix if it breaks free and starts spinning again - the shaft runs in floating bearings, so for it to stop turning, the bearing has seized to both the shaft that spins in it, and to the housing that the bearing turns in. Chances are just one will break free, so it'll wear that one surface in a hurry - which means keep an eye on it, once the bearing wears enough that the compressor vanes hit the housing & gouge it, you'll need the whole turbo, not just the core.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina