I use conductive paint to fix them. It isn't easy, but it can be done with some trial and error.
You need to scratch the outer tire and bronze strip surfaces clean with sandpaper or an Xacto blade, and then apply the conductive "goo" across them, and you MUST do it at the tire edge AND inside, near the axle (both ends of the bronze strip). The inner end is hard to access with the goo, but it works.
I've used MG Chemicals Nickel Print, a little jar of thick liquid epoxy with nickel powder suspended in it, and later, a "Circuit Scribe" conductive silver ink pen, where I just dab the pen point all around the area to make a pool of conductive silver ink, and then I can use the point of an Xacto to spread the pool the way I want it.
The Circuit Scribe material works better. I could get the resistance between the axle and wheel darn close to zero whereas the Nickel Print might be 0.5 to 2 ohms. They don't sell Circuit Scribe as a liquid in a jar, only as a pen.
https://www.circuitscribe.com/. They are $10.
Circuit Scribe also has a new "Beta" pen product for $60, that has two pens: one with a spring-loaded ink chamber to make the ink flow more regularly at the point, and another that you can squeeze to make the ink come out like a paint pen. The Beta is sold out, unfortunately. I'd really like to try that.
They used to sell "resistive" ink pens too, available in 100 ohms/cm, 1000 ohms/cm, etc --- a variety of resistances. I always thought those would be great for making detector wheelsets, but alas, those seem to be gone from Circuit Scribe's website now.
Anyway, I've fixed a bunch of old Mikado drivers with this stuff, and they do work. You just have to be persistent and use an ohmmeter to make sure you get continuity between the axle/bearing and the wheel tread. Sometimes, I have to scrape the stuff off and try again a few times before I get it. But once they are fixed, they seem to last. I know I've checked some that were "fixed" over a year ago, and they still were conducting properly.
