I think your first step is opening it up to assess the damage. You might be able to rebuild the motor. If not, bolt the 4.4 in there and fire it up. I'll bet it runs on the 4.6 ECU.

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2002 X5 4.6is - Titanium Silver / Black Alcantara - Dinan Stage 2 - Bilstein/Eibach - Brembo BBK ----SOLD---
1993 318i - Brilliantrot / Tan - bone stock daily driver