Hello, I'm working on a motorcycle where a dodgy speedometer and cable has now damaged the gearbox leaving me with no known-good components in the speedometer system and I'm thinking of replacing the whole lot with an electronic speedometer and sensor. Hopefully some people here will be able to make some suggestions or talk me out of doing something stupid.

My life in motorcycles has been a bit haunted by mechanical speedometers going wrong so I'm kind of excited to have the chance to throw the whole mechanical system out and go to something with fewer moving parts. I'm looking at buying a third party speedometer with a passive sensor that will be able to detect magnets attached to the wheel, but can't quite decide on how I'm going to attach those magnets and don't want to commit to buying an expensive speedometer before I have a reasonably good idea of what I'm doing.

The old gearbox is made of plastic and cracked and broken in so many places that I think it's worth declaring dead:

I'm thinking of replacing this with an aluminium housing which I'm fairly confident would be easy enough to make on my ML7 lathe. I could then drill a hole in the body and braze on a tube which could accept the electronic sensor. The problems start when I start thinking about how I'm going to attach the magnets the sensor is going to detect. The part that is going to rotate with the wheel is that geared ring also seen here:

That mates with two notches on the hub and rotates freely within the housing.

That's what I'm not sure about. Aluminium doesn't make great bearings right? So trusting two aluminium parts to slide freely against each other if they ever meet doesn't seem too wise. I also want to try to keep the road salt out of the system with a strip of felt or something.

I feel very stuck deciding on a way to attach something to the hub, the powder coating seems like it could be a problematic surface to affix something to. If I could affix something to it though I could embed the magnets in that and it would never have to come close to touching the body and sensor if a couple of millimetres tolerance were allowed. The 2.95mm thick alloy wall doesn't seem like a good, easy or even smart thing to drill and tap into, and I can't have bolts protruding really either.

Does anyone have any ideas for ways to affix something onto or around that part of the hub? Perhaps I've run into a dead end and should forget about driving the speedometer off of this side of the wheel, concentrate on making a cup to act as a dummy gearbox to keep the salt out, and drive the speedometer off of magnets affixed to the brake disk like everyone else?

Or perhaps I should shell out some extra money for an active sensor that might be able to detect those notches on the hub foregoing the need for magnets entirely?

Thanks.