I've been thinking about fixing one of these Denso units for years but always ended up with used replacement or in last case a brand new unit that then failed within 3 months and was replaced with another brand new unit. I kept the original alternator though and finally had a chance to take it apart and see what happened, Disassembly is fairly straight forward, the top cover comes off and there you can find the bridge rectifier, voltage regulator and brushes.
Brushes can be of course visually inspected, mine still probably had 50k miles left in them, the bridge rectifier can be checked with a multimeter(diode test) and of course that was the part that failed. There is no easy way to test the voltage regulator so that will also get replaced. The big rear bearing was completely toast too but the front small one was still ok(it will be replaced anyways). The windings can always be checked as well but these are I imagine always ok. The only other thing that might need to be replaced is the slip ring which is a bit of a pita to replace as the wires are covered in epoxy so if your slip ring is indeed worn, the epoxy needs to be chipped off and the copper ring cut off the shaft, then you can press in a new slip ring.
The big bearing is fairly easy to just bash out with a socket and hammer but the small one that sits in the front cover, ideally needs to be pressed out of the casing and then bearing pulled off the shaft with a puller. I don't have a press so ended up cutting a hole in some thick mdf which was exact size of the commutator with the case sitting against the MDF plate and hammered it out, then pulled the old bearing off with a puller.

Since there seems to be zero information about parts, thought I'd mention what's needed for this repair.
Big bearing - 7x52x16mm
Small bearing - 15x35x13mm
Voltage regulator - CARGO 134447 , DENSO 126000-0430 , DENSO 126000-1200, IN435
Bridge rectifier - ASPL ARC6017. HONDA : 31127PY3003, HONDA : 31127PY30030
Brush holder - Parlex 133673(but there are lots of them)
Slip ring - I didn't get as mine is still ok. They are widely available too.

The cost of all these parts including slip ring is approx £80 GBP so I think well worth it. New Lucas alternator I think is currently ~£350. Few pics too

 

I found the sourcing of the IN435 voltage regulator iffy to get the correct one or one that works on E - Bay

In the USA at the auto parts stores they have an alternator machine to spin up and see VR and all before car installation

 

https://www.maniacelectricmotors.com/ also sells a rebuild kit as well

 

All the parts finally arrived and were fitted today, all went well, I've hammered the small bearing onto the commutator, put it in the freezer and the housing in the air fryer for 10 minutes at 200C and it went in with no issues with a single tap of the hammer, gotta love air fryers Big bearing was even easier to hammer in and the rest of the parts are screwed in. All good and now I'm wondering if I can be bothered to fit it back in and replace the current chinesium in there.

Of course I had to disassemble it again as forgot to put back the small rubber isolators between the windings and the bridge rectifier.

 

Fitted alternator today and all is working well


The chinesium alternator that's been there for the past 12 months does work but 1st, same one failed within 3 months before I replaced it with this one. My biggest issue with it is the the much thinner main post, that copper bolt is half the thickness of the original one and at full load I can kinda see the front lights very slightly pulsating. Pretty sure its down to post being too small. I will probably keep it as a spare though and a bridge rectifier with a bigger post like the one I've fitted to the old alternator can be fitted too.
Tested this one at full load too and its rock solid at just over 14V.