It's possible to alter an SKS trigger to improve it quite a bit. Fairly simple job, but it requires an intimate knowledge of the function of the SKS fire control group.
Shortening your sear, being careful to maintain proper engagement and face angle, will help take some of the creep out of the trigger. Polishing the engagement faces (NOT FILING) can help eliminate felt creep and trigger pull, at a minimum it can improve your trigger feel and drawstroke. Lightening the mainspring can reduce the pull.
Messing up on any of these can cause your rifle to fire full-auto uncontrollably, which would put you in a world of hurt with the feds for producing a fully automatic weapon.
I can't tell by your post whether you 'want to keep the original parts' as in you want to alter the original parts and avoid buying new/aftermarket parts, or you "want to keep the original parts", as in your want to be able to remove the originals to restore the rifle to it's original state at a later time, so you're wanting aftermarket parts?