Short answer
Gather evidence before quotes: mechanical condition, corrosion in the cooling and exhaust path, parts availability for your engine, and the state of the installation. Then compare the repair against the full repower cost including beds, exhaust, controls, wiring and commissioning — because those systems, not the engine, dominate the bill.
You will need: Survey findings, oil analysis, at least two written quotes
Age alone does not decide it. Well-maintained engines of considerable age cross oceans; neglected young ones fail in the first season.
Gather the evidence first
- Mechanical condition: compression or crankcase testing where warranted, and oil analysis trends across several samples rather than one.
- Corrosion: the exhaust elbow, heat exchanger, raw-water circuit and anything aluminium below the waterline.
- Supportability: can you get filters, impellers, belts, gaskets and a starter for this engine where you intend to cruise? In Australia this is a bigger factor than owners expect.
- Installation quality: mounts, alignment, wiring, fuel system and ventilation.
Then compare honestly
The trap is comparing a repair bill against an engine price. A repower is beds, alignment, gearbox and drive compatibility, propeller match, exhaust, cooling, ventilation, fuel system, panel, wiring and commissioning — and those frequently exceed the engine itself. See repowering a boat and engine installation.
If the surrounding systems need work regardless, that shifts the calculation towards repowering, because you are paying for much of it either way.
Do not assume more power
More power is rarely a straight swap and rarely produces more speed on a displacement hull. See can I install a larger engine during a repower.
Compliance and insurance
Changing installed power can affect plate information, registration and survey status where it applies, and your insurer may reasonably want to know. Confirm before committing — see marine engine rules in Australia.