i had a mysterious petrol smell on mine soon after i bought it. turned out to be a clip on the fuel hose to the petrol filter not quite tight enough. the leak was so small that the fuel was evaporating in the engine bay heat so there wasnt any telltale stains anywhere to be seen.
If you've had a new engine, even with new hoses & clips. I would be inclined to check all fuel line clips, changed/disturbed or not, just to be sure.
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