Don’t know if you’ve already found your solution – but the answer is not that straight forward.
When you say ‘stock’ SIG stock, there are several –
1. Original plastic (Gen I) folding/collapsible – This is considered by many to be the worst of the lot. Cheaply made, the hinge is plastic and long term durability is suspect. Typically can be purchased new on Gun broker for approx $120. You will need to add a standard M4 style collapsible stock (MAGPUL is common) of your chose.
2. ACE manufacturing has a range of top quality machined aluminum parts to convert to any style (left or right hand fold) stock. Again, any standard mil or commercial spec AR baffle tube and corresponding collapsible stock can be used.
3. Original collapsible only – these are nothing more than standard AR M4Gray collapsible, install one of your chose.
4. New style (SIG 55x copy), these are only found on the new style ‘classic’ lowers. If you wish to convert to this style, you will need to purchase a classic and swap out the complete lower – not a cheap conversion.
My 556 Classic SBR with AAC M4-1K: Started life as a 556P, swapped out the lower from a SIG 556 Classic.