I am unaware of any aftermarket parts for these gems...
I adore my 64's. There is very little that can be done to the trigger other than to fire 10,000 rds through it. "Natural polishing of working parts"
If you're extra careful, the trigger group itself can be minutely shimmed away from the receiver. This must be done with very small changes. Shimming the group too far away from the receiver can leave it in a very dangerous condition. Make certain it passes bump tests before you put ammo anywhere near it, and test fire with 2 in the magazine. This is hack work, and if not done correctly not only can it make a firearm unsafe, you can also damage the trigger group by rapidly wearing down the sear/resets.
Be safe, have fun. These rifles have been in production for over 50 years with little change for a good reason. No aftermarket stuff to slap on isn't always a bad thing.