You can use the existing wires and sensors, just need to change out the end-of-line resistors to match the new panel. Nearly all systems and sensors operate on a 12 volt system. Depending on age, some sirens are actually just speakers with the siren driver in the panel. Hook up the siren directly to a 12 volt battery to verify.
If it is an Ademco panel, entering the programming section can be done by powering down the panel, hooking it back up, then hiting the #and * button at the same time for a few seconds. If #and * dont work, try the 1 and 3 buttons.
If you are in need of a new panel, I can send you a new panel, keypad, battery, transformer from DSC, which is my favorite panel to install. I could program the panel for you here, and send it to you. You would have to replace the eol resistors and program the time and date and user codes.
I own a security alarm business, been installing for 14 years now. The resistors are what make the panel know if the circuit is good, the sensors let it know if a door or motion detector is open.
Outside sirens are great if you have neighbors, and monitoring does not need to be hooked up. I charge $15 per month for monitoring.