I would adjust the valves and check compression before condemning the engine. How bad is it smoking? How much oil does it use? I'm guessing there's a little life left in it, but after over 700,000 miles, I wouldn't expect much.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.