Hope the son won't be getting the car back anytime soon

I'd replace the engine. IMHO there's little that will replace an E39.

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'99 528i ('98/12 build). 174,000km BY29428/Royalrot
Breaking My Wallet since 2009
Mods: Stoptech SS brake hose, 280piece toolkit resting on trunk floor, Beisan VANOS seals
'99 540i (grandfather's)