The ZW breaker was suggested to replace the earliest Z breaker if you can find one.
A modern automotive self resetting thermal breaker will work. They are faster acting, than the originals too. You usually have to order them though. NAPA has a broad variety to fit 5a and up. I think there is room for a manual button reset one if you want to drill a hole, but bakelight is brittle, you need a bit angle that won't bite and crack it. Grinding or with a Rotory file, safer. The external ones are a good idea added to it as a secondary.
Pop the top and inspect the variable windings for wear or sawing from broken rollers, just like a ZW. Worn windings greatly reduce output. Overheated arms turn blue/black, also weaker. Flat rollers are a pretty easy fix, but order extras and extra rivets, there is a slight learning curve. (Even with extras it's still under $5 I bet.) Rivet over crimped bends, and roller won't spin, or roller gets broke during crimp. (Graphite is very brittle)
Check the gears and looseness/ wobble on shafts as a clue to wear. There are two handle types, one has pins, the other just slides on.
It's basically a ZWs equal in a different configuration. Its "daddy".
The extra 5 volts is sort of eaten up with a prewar whistle controller. I'd use a diode string or beefy triac vs those old control boxes. Maybe gut one and shove the modern stuff in for looks. You'll be missing the 5v boost saved for the draw of the old whistle motors though, so the loco will slow some unless you work that into the thing, which you can do, leaving you with 18-20v like a ZW and a 5v boost for an active whistle. You can also add a switch to reverse diode polarity for bell function on modern stuff.
A fun KW note: My first kw had gears inside for one handle like a Z, and both the handles worked as away for on, to you for off. My set was an X set, and likely had a development prototype build thrown in the box from the development room by mistake. It died, and my Grandpa about crapped himself when he opened it to inspect it. He died about a year later and the "pal" the collection was sold too, took lots of stuff he wasn't supposed to, including that. Not to mention only giving Grandma a very small fraction of the collections actual value. (It wouldnt have covered the GG-1s alone, especially considering an unopened shipper with a year one blackjack, bought new, was there too) So look hard at those old kw shells under show tables, the right hand voltage numbers are reversed on at least one. I just want a photo to prove it exists)
