Good question. I think both ways work here, the given suggestion and yours.
There is a different nuance, anyway.
"Il motore è mio, non tuo." = The engine is mine, but I could have more engines somewhere
"Il motore è il mio, non il tuo." = Both me and you probably have only one engine
Cool, thanks. To me Italian seems to be more flexible than the lower levels on Duolingo suggest. Fortunately :-)
A good challenge statement that would probably never be heard in the USA or Italia.