Yes. You will need a mechanical system if you are talking about the boat in your profile.
1. Stargazer Master Module.
2. Perfect Pass Wide Display (Gauge).
3. Wiring Harness.
4. Mechanical servo to throttle cable kit (fairly boat/brand specific).
5. GPS Antenna
Most likely, the parts you could source on ebay or ski-it-again will be the gauge and master module. Unless you get lucky and find one for sale, the rest, you might as well buy from Perfect Pass or one of its dealers.
Most used SG systems come from people who have converted over to Zero Off. Unfortunately for you, those are going to be for newer DBW boats. So the only thing you could use would be the Gauge and antenna.
I sold my original SG module last year when I upgraded to zbox, which didn't work with some earlier SG modules. So you might get lucky and find someone selling one for that reason.
Previous: 1993 Prostar 205
Red 1998 Ski Nautique, PCM GT40, 310 hp, , Acme 4 blade, Perfect Pass SG/Zbox.
Be kind. Have fun.