A compression test may help you find your problem. A leakdown test will help more. Do both and you will very likely find the problem. My first guess is valve guides.

I used to have a Suzuki Samurai. The little 1.3 litre four banger in these was usually good for about 130,000 miles before overhaul. Mine had 180,000 when I bought it. It used rather a lot of oil, and smoked a good bit, but still delivered good fuel economy and as much power as you could ask from an engine originally rated at 60 hp. The person I sold it to did valve guides shortly after and it ran just as well while no longer smoking or using much oil.

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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.