A kit, I think, which at 14 would be an appropriate level of effort and achievement. Disembowelig a commercially-manufactured clock would in general yield a more compact result and be less fun (and he wouldn't be much moved to show it off to his scientifically literate instructor); and designing and building from scratch would be beyond nearly all 14-year-olds, not because they lack aptitude but because they lack the necessary mathematics.
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-- Carl
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