I presume the product is trans-1,2-dimethoxycyclohexane upon opening of the oxiranium ring. My comments were not directed to providing a substitution product, but what one should reasonably expect. You would also get trans-1,2-dimethoxycyclohexane if the reaction were carried out in methanol without NaOMe. It would be easier to predict that as the product as those conditions are more easily understood as SN1 conditions. If the reaction is an SN1 mechanism, then NaOMe is not a kineticly important reagent nor a catalyst. It may well have been present, but in this case misleading to predicting the product.