A replica is a model of something, but it is a particular kind of model and the two words are not interchangeable. A model may represent a type of thing rather than a specific example: a model railway, and it may be quite crude and not very much like the original: a pink plastic model of the Taj Mahal. A model may be of something which does not yet exist: engineers sometimes make detailed scale models of things they are planning or constructing. A replica is an exact copy of a specific thing which exists or has existed in the past, and is usually made to scale.