Can you get high capacity/high pressure oil pump for that engine,if not shim relief valve for a bit more oil pressure to feed turbo and rest of engine better.
Turbo's fail because people never let them spool down after hard driving and it was probably well used prior to you,bit of a bugger after all the graft
Rebuild kits are available but would you trust something that's got to run at silly revs and heat without being set up properly, No.