Can't help with aftermarket quality, but a sealant I'll throw in a good word for Yamabond, I've always had great luck with it. Only negative-- it is a drag trying to get it off if you ever have to go back into the motor, the stuff is tenacious. When I was racing motocross I had a small leak in a two-stroke case. Just for giggles I smeared a little Yamabond on the outside to see if it would suck it in and plug the leak. Damned if it didn't, which saved me from having to split the cases. Judging from the positive responses on Threebond, I'll definitely have to give that a try.
Some of the non oem can be a pain to remove, I find the threebond fairly easy to remove and well priced so little reason to use anything else.
Have a look at the KTM sealant, that is hefty overpricing at 80 something euro.