After working on a Kustom K100-2 with broken reverb, I've eliminated one wiring problem and discovered that the short reverb tank that was gerry-rigged into place by the previous owner needs to be replaced. When I substitute in a good vintage tank, the reverb works fine. I've seen a number of threads about a good modern replacement but the threads arent't consistent. What modern reverb tank would be a "drop in" replacement for the original tank (or close at least).
I would buy a vintage one from Ebay, there is nothing like a good USA made spring reverb, Accutronics, Hammond/Gibbs are a couple of brands, I would avoid china made mod news stuff personally, like all the newer stuff, it has no soul