put in the new buckle and then stitch the strap back up.
A little too much work IMO so I'd just get a new strap and buckle
Cheers,
Anthony

he already got a new strap and just missing the buckle.
Pepe, just measure the buckle end of your new strap carefully and get a new buckle that fits. Panerai do make 24mm & 26mm polished screwed-on buckle in same style as the buckle that came w/ your California. Needless to say, if you choose to go aftermarket, there're plenty around. Perhaps you can even check w/ the vendor/shop where you bought your strap?