Choosing the right woofer involves a fair amount of mathematics. The cubic footage of the speaker box and port(s) if any, crossover points, etc. all have to be accounted for. In addition to the wattage and frequency response, you'll notice specs like Qes, Qts, Vas, and many more on sites that cater to speaker builders. All of those numbers and more will have a dramatic effect on the resulting sound of your speaker.
You might want to try a site like
Parts Express, which is one of those sites that started out as a site for people who build speakers. A call or email inquiry might save you a lot of guess work. They may be familiar with your speaker and/or have a suitable cross-reference book to make sure you get something that will fit your specific cabinet in more than just diameter.
Your idea of buying 2 matching woofers, is almost certainly the best way to go, at least they would have the same characteristic strengths and weaknesses. If the tweeters and crossovers work, personally, I'd be inclined to keep them as is.
EDIT: And if PS10, is short for "10-inch Powered Sub" you're right back where you started from - unable to utilize a vintage amp without doing some modifications to the sub.
Good luck.
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