On my 716i I replaced the charging handle with the ambi one from SIG and I put JP 4.5 lbs springs in the stock trigger. Fun gun to say the least even without those upgrades.
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A google search shows it being at the website listed in the url, I have never used them nor do I know if legit or not. Shows as being in Montana for a physical location.
http://www.selwayarmory.com/sig-saue...num-black.html
I understand the desire to keep it all Sig, but you'll likely be better served by BCM charging handle.
call Sig, they have an ambidextrous one, don't think it's site listed.