best replacement woofers for early advent speakers

my dad has these old advents that he bought new about '71 he says, and the surround on the woofer is gone again. i'm thinking we should invest in a more modern pair of drivers for these speakers, but i don't know where to look. i don't want to ruin the original sound quality. i checked out simply speakers but noted that others on here said their replacement woofers are not the same quality. i'm also unsure about self-repair kit. will that be true to the original? i love these speakers and i want to upgrade them with modern technology but i also want to preserve their originality to some degree. is there a product out there to suit my needs? these things sound killer when they're healthy and i know if i treat them right, we'll get another 30 years out of them no problem.