Tune up parts? When was the last time you did plugs/wires/cap/rotor? I'd pull a plug or two first and see what they're like.
After that I'd figure oxygen sensor. How many miles are on that?
Personally I'd say 23-24 is pretty darn good especially if you're doing 80 on the highway a lot and the way the m103 revs high at speed. I'm lucky to crack 20 with new york driving lol. Unless you're used to getting better than that with your car.
Also the green 2-pole temperature sensor at the rear of the head, maybe give it a test?
Unless your cat converter isnt really scrubbing anymore and making the exhaust smell rich. I doubt that though theyre pretty tough.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C