I think the reason you see so few AC-30 clones is that it's a ridiculously complex circuit to replicate. Most just pick and choose their favorite Vox bits and mod to taste, ala Matchless DC-30. The beauty of the tweed Fender circuits is that they tend to be just about as simple as can be, which means you can focus on components and tuning more readily.
The 18watt kits are popular and an okay value due to a couple of virtues - they sound great and are subtly different from other amps, they are lower wattage, which is desirable to many, and they are rare as hens teeth, so the expense of a kit is nothing compared to the real thing. If I had the money, I'd buy a kit. Since I don't, I'll do something else. I even balked at the Mission 5E3 kit because I realized it'd be louder than I wanted. (I need to start playing out with a drummer before I start listening to the Indigo Girls . . .)
Love to see the original circuit on that Supro. I'll bet it's yummy.
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