flyson,

Get a new one. Don't bother with any leak sealant etc. They won't hold and if they do it won't hold for long before it pops. Last think you want is it to burst while you're out somewhere far and left stranded.
Then you'll need to buy a new radiator, pay for towing, the hassle and if your lucky you won't need to rebuild your motor.

Get an all aluminium one. They will last forever.
Install it yourself. It's easy. When refilling have the front of the car tilted up so the air bubbles can escape through the cap. Also have your heater on to let the coolant flow through there also.