I changed the head on my boat. It took a few hours and was simple to do. The plumbing was all in place, all I had to was remove the connections to the head (2 hoses), remove the head (4 bolts), drill 4 holes to mount the new head, bolt it down, and connect the 2 hoses.
If you don''t have a holding tank or hoses it will be a lot more complicated, but it can still be done. If you have the thru-hull for water intake, and head pump out (if legal in your area) it will be a lot easier.
If you don''t have the thru hulls, I would just use a good porta potty.