You can do whatever you want in the privacy of your own home. It's when you decide to start distributing that remake that issues arise. And yes, you would certainly need permission to sell a remake. When the remake craze started years ago, nobody had permission--everyone basically flew under the radar and hoped for the best. AGDI got approached by Vivendi, but it was after they already had released their first remake, so a "fan license" deal was reached to allow them to continue working on the various projects they had already started. TSL was shut down twice, and managed, through a lot of fan petitioning, to get Activision to work with them to grant them a fan license, too.