There really isn't an upgrade for SKS triggers like one finds for AKs and ARs, etc. Best thing you can do is send the trigger group off to either Kivaari or Murray's to have it tweaked. They can reduce creep and pull weight to give you a crisp clean positive break.
I've always used Kivaari for my trigger work, but Murray's does good work too. They both have sub-sections here in the retail/vendor section. (Retail and Business)