A full system includes the engine and manifolds but not the risers. The risers are where the raw water is mixed with the exhaust gases to cool them down before they exit the boat. If you add the system now you will need to flush it several times as rust and scale will come out of the engine and collect in the heat exchanger. IMO I would not add it until/if you repower.
1997 2400 SCR
5.7 Vortec / Bravo 2