Mechanics are not protected by U.S. copyright. A specific description of those mechanics (aka "text of the rules") is.
Monopoly is a TradeMark and Bonopoly is (I bet a jury will agree) confusingly similar.
The line is, if you get the attention of someone big (or big headed) they will sue and or threaten to sue. Even if you are legally in the right it will probably not be in your financial interests to defend yourself.
So,
- Don't be a dick.
- Do not use any names, art, design from the original.
- Don't do this for profit. It will never be profitable.
- Make your electronic version, but be prepared to dump it when you get cease and desist order.