To get information on the OEM manufacturer of accessories provided with Boston Whaler boats of recent manufacture, contact the Boston Whaler customer service department. The Boston Whaler customer service department is staffed with extremely knowledgable people who have decades of experience with Boston Whaler boats, and they are the most gracious and helpful people you could ever encounter. I am sure they can give you advice on the marine toilet that was installed originally in you 1999 Boston Whaler boat.
Also, check on the website
http://whalerparts.comto see if there is any information about your particular boat and model year, or something really close. Often you can discover lots of information there. However, once you discover the OEM component, you will have to hope that it is still in production if you were planning to buy one.
As for how the system that you no longer have installed was intended to work, I don't there was too much out of the ordinary about it. The owner's manual for your boat should help explain the system.
A boat made in c.1999 would very likely have been fitted with a marine sanitation holding tank. Does the 1999 230 OUTRAGE boat have one now? If not, you will have to install a holding tank in order to comply with no-discharge rules in effect now just about everywhere.
To learn more about marine sanitation system, I recommend you visit the website of a manufacturer of marine sanitation devices, THETFORD. They offer a great deal of information. See
http://www.thetfordmarine.com/us/They also have many newer designs that might be preferable to re-fitting your 1999 230 OUTRAGE boat.