Your best option that is somewhat reasonable in cost is the KRG Whiskey 3 chassis . Their folding stock is made like Swiss watch. I have one on my TRG-42 ( anybody want a green OEM stock ! )
The machine work, fit and finish is impeccable . Only issue is it took forever to actually get one
Second option for Tikka is the XLR Industries Evolution chassis . The folding mechanism is made by Ace Stocks . They do what they are suppose to do but is not a nice as the KRG . Advantage of this system is its actually inexpensive for what you are getting
Lastly, a T3 action will not drop into a TRG stock ( I tried ) . I just got use to the cheap plastic stock of my Tikka Tac
If you actually price a replacement TRG stock you wouldn't even consider using that. The TRG42 folding ( not including forend ) is $2900 @ Brownells WITH gunsmith discount ! ) The non folding OEM is about $1500