A few thoughts on Sachs engines:
Chicago 7915 seals can be used as main seals-run two back to back on the primary side(extra protection)
The selector key has a "drive" side and a "coast" side. The drive side wears much quicker than the coast side. You can get extra life by rotating the key 180 degrees on the selector rod. The right solution is to replace it but they are getting hard to find.
Same with a few of the gears on the main shaft. They are reversible-look for wear on the drive side.
The clutch hub on the layshaft should be installed with 271 locktite as well as the small primary gear nut.
use locktite 241 on the selector arm pivot bolt- they have been known to come loose.
keep your kickstart shaft splines clean and the lever tight-they have been known to strip.
keep the shifting parts shimmed up as tight as you can- the more slop in the works means lousy shifting.
I use 80 wt hypoid gear oil and have not had any clutch slipping problems.
No way Locktite the two items you mentioned, sele3ctor key and pivot screw only. The primary gear nut has a lock tab. Proper torque on the layshaft nut and they won't come loose.
Normally I would agree with you but I have had both those pieces come loose even with the proper torque.In my experience a few drops of insurance goes a long way. No trouble with disassembly BTW.
the lock tab will keep the gear nut from spinning off but it is tied to the gear instead of the crank. If it is not tight it will "rock" back and forth and will hammer the keyway on the crank.
Granted, the clutch hub nut really has no place to go if it gets loose, that keyway can take a beating as well.
This has been my experience.