There are several elements to a game... and most but not all are protectable.
Exactly which varies widely from country to country.
Some games that are explicitly legal reworks in the US are proven infringements in France, for example.
You need to look at where you're writing at, and intending to sell to.
Even where it's legal, plagiarism is still generally considered unethical; make it different enough to be worthwhile.
In the US, the literal text is protected by copyright, as is the board art (and by derivation, the layout of spaces). The Title is a trademark, as are certain artistic elements. The Chance and Community Chest cards are trademark and copyright. The specific property names and the rent and prices are pretty much copyright. The process of play was Patent, but the patent is long expired. And, given later reforms, the flow of play is no longer protectable.
However, remember: I'm not a lawyer, and I'm surely not YOUR lawyer - engage a good Intellectual Properties lawyer before publishing a derivative. Find out what your local rights are, and how far your can go.