I saw his spleen on display in the Armed Forces Institute of Pathology museum. While one can survive without one's spleen, the surgery to remove it in 1865 was highly dangerous.
Back in 1993, an anthropologist discovered Lewis Powell's skull in a drawer and it was returned to a family member for burial in Geneva, Florida.
A fellow researcher friend of mine, Mike Kauffman has been researching the Lincoln Assassination for over 30 years. In 2004 he published "American Brutus: John Wilkes Booth and the Lincoln Conspiracies." I highly recommend it for those interested in the topic.