How much will it cost to rebuild a 1991 thunderbird sc engine... OR how much will it cost to switch the engine

guy is selling a 91 thunderbird sc for $900 ... its in running condition and the only thing wrong with it is there is a rattling noise in the back end.... he said it was the rod. the car is nice... fully loaded, moonroof. blah blah... is it worth it to swap the engine or rebuild it.... i need some prices.. and what one out of the two ideas is best?????? THANK YOU !!!!! :D