I got a few sachs engines. I'm going to share my experience with the manual clutches first and then give some tips on improving the automatics. This applies to 504's and 505's and assumes you have the standard 2-pad/3-plate clutch
my bike runs a 2 speed manual and I had a load of trouble with clutch slipping. Then I got some milled pads and plates, the german super clutches.
only problem is they're super expensive and you cant get em shipped stateside so you gotta know someone over that side of the pond.
They rip. no slip. if i revved it and dropped the clutch, it'd pull a backflip out from under me.
anyhow i wanted to share about em, they wouldn't be too bad to replicate.
New "uber" pads [#18], 2.15mmx4
plate [#16] 1.7mmx5
17.1 (meas 17.3) total thickness
plus it has a stiffer base spring [#20]
old pad 3.2x2
plate 1.7x3 (on an standard 2 pad automatic clutch you get one 1.7mm plate and 2 2.6mm plates, the 2 pad manuals have three 1.7mm plates)
spacer [#15] 4.5mm
16mm total thickness
and on the manual clutch you also have 3 15.8mm (3mm diam) pins [#14]
If you find any second clutch pack even from an automatic, grinding down the clutch pads is all you'd need to do. 1.7mm plates should
be thin enough as-is, the manual's already have 3 and the automatics usually have 1. Pads are easier to grind too. I'd grind the side where the oil channels don't go all the way out the teeth, you'll see what I mean looking at em, so that you dont loose oil turbulence. it'd be important to keep the 2 sides absolutely parallel too
Once it's all down to those uber clutch sizes though, it still doesnt quite fit. You can mill the bottom spacer [#21], theres a lot of room for more clutch to fit in the bell, but the spacer pushes it out. that’d be best. You also have to ditch the spacer [#15] on the end.
or another option, instead of milling down #21 you can space out the cover. if you just flip the inner spacer  and get rid of the outer spacer it buys you enough room to fit it all in the bell, BUT it pushes the engagement bell piece [#11] too far out so the clutch cover disengages the clutch when it’s tightened on. a few extra gaskets or a custom spacer works. So either you gotta mill down #21 about .5mm or flip it and add some extra gaskets on the clutch cover. I did the latter and have 3 clutch cover gaskets.
problem 2, the new uber clutch pack is too thick for the old pins. the clutch disengages when you pull it by pressing the set of 3 pins  thru to push the back plate and spring [16,20]. The pins are only 15.8mm which works because the normal outer spacer  is recessed. with the new setup at 17 thick, you need longer pins. i found some (3mm diameter 18mm long) from McMaster Carr. they work, I have a ton extra. now if you want some, they’d be free.
so. longer pins, flipped inner spacer, no outer spacer and a few extra gaskets for the cover, and it works. I run a belray gearsaver 85w gear oil. Tried everything down to ATF with no change, and sachs recommends 90w for it’s manual tranny bikes.
I also put a little whirly piece to whip the oil around and keep everything lubed and cool. that's equally or more important than friction area. it's just a cut piece of tin can with bent out tabs bolted in there. pins go through it. i havent any pics but get creative.
For you normal automatic folks:
This is an Automatic sachs clutch. I took parts from two standard 2-pad/3-plate clutches. milled the pads to fit 3 pads and 4 plates for more friction area and less slip. The 2 center plates are the thinner 1.7 (usually one in each clutchpack is this size), outer 2 plates are standard 2.6mm thickness. pads are milled down to 2.3 from 3.4mm. Also good to recut the oil channels in the pads. After that you just have to add shims to make sure the center toothed piece [#19 above] has proper clearance so it grabs only as the donut springs out or the starter bell is pulled in. ready to rip.
The other big problem people have with the sachs auto clutches is the nut backing off the end. the reason for that is that the clutch only attaches to the crank by the nut on the end, whereas most clutches are taper fit so they really get snug on there. That means the sachs clutch can spin the nut free, basically because the clutch can spin freely from the crank if the nut gets any loose. Taper would be ideal, a woodruff key would be great but neither are really easily doable.
What I did was to take the toothy bit [#19]and drilled and tapped a 1/8 in. hole through the side of it and then got a short allen set screw for it.
-somethin like that.
I lined it up on the crank and drilled a tiny divit into the crank where it would sit normally, making sure that's alligned right. Then drilled a big hole in the top of the clutch case to get to the screw and tighten it down through the slits in the side of the bell once the bell and spring and evrything behind #19 was in place. Then i just put a cover over the top hole, theres no pressure on it, just to hold the oil in.
Assembled the rest as usual. Red loctite on the end nut is
still a good idea. clean it with brake parts cleaner 1st to get any
Hope all this info can be of use to someone else.