"copyright law with regards to fangames is contingent on whether or not it is being sold" isn't true at all! Some companies have a general policy of drawing the line there, but Nintendo for example has gone after free fangames many times.
Not a lawyer, but doing anything potentially iffy with Tetris seems particularly risky. https://en.wikipedia.org/wiki/Tetris_Holding%2C_LLC_v._Xio_Interactive%2C_Inc. is one of the most extreme cases ever of a game publisher successfully pursuing a copyright claim.