From a 550? No, because a 550 (and all the Honda SOHC4's) uses a field coil with a variable current to create more or less output from the stator windings. Your CL125 (I'm assuming you mean the single, not the earlier twin) uses a permanent magnet rotor which, of course, cannot be varied.
There's an easy way, and a hard way to do the conversion.
The easy way is to simply swap in a 12-volt battery and change all the lightbulbs. But, this won't charge well (at all...) at idle and low rpms. It won't really do it until 4000 or better rpms. I did this on a 1978 CM185T Twinstar commuter (which uses the nearly-identical overall system) and it worked perfectly for years. But, my commute was 30 miles of 55 mph steady speed (7~8k rpms), with nearly zero stops. So, my experience may be different.
The harder way is to swap in the stator from a CB350 twin (along with 12-volt battery and bulbs, of course). I've never done this, but I've read several write-ups that says it's a bolt in. The 350 stator has more winding wraps than the 6-volt singles, but apparently there is enough room inside the 125's stator case for the fit.. You'd also want to graft in the regulator (or an aftermarket version of that style) from the CB350. The CL125 (and all the 6-volt Honda singles of that era) did not have a regulator at all, but just depended on a "matched" system (design output = design load). As for a rectifier, any modern silicon single-phase, full-wave bridge rectifier will do. That is, two AC inputs and two DC outputs (a + and a -). You can buy such a thing for about 5 bucks online or at Radio Shack.
The rectifier swap is ALWAYS a good thing because the early Honda rectifiers were selenium (the ones with the large orange plates) and selenium is not as efficient as silicon, and you can pick up some extra charging watts just by eliminating the selenium loss. And, I"ve read that selenium, in addition to being less efficient right from the get-go, also degrades over time making them even WORSE.
The hardest (and best) is to rewind the CB350 stator to eliminate the "dual" charging windings for a single, continuous winding. This improves stator output. All the rest is the same as above. There are several good write-ups with pictures and details online here and there if you're interested.
And lastly, I've read that the later 70s (say, 77 ~ 79) rotors used stronger permanent magnets than the early 70s models, so if you install one of those later rotors on your early model, it boosts the output as well.
So, that's all I know about it.
Good luck, whichever route you pick...
Kirk
