There are many choices for this. but you should plan to purchase a 3- or 4-channel (bridgeable) amp to run a right, left, and subwoofer channels. Replace the door and kickpanel speakers with component speakers and throw away the rear speakers and mid-tweeters in the doors. Of course, you already have the subwoofer. Modern speakers and amplifiers are so much better that you will not be disappointed.
You will want something like 80 watts x2 for right and left plus 200-240 watts for the kicker. Open top roadsters need some power.
Near the top of the forum page, on the right, is a button labeled "Search This Forum." Click that and type "amplifier" and you will get dozens of threads to look through and consider.