There is a reason why you won't legally be able to do that. Out there somewhere is the ATF letter that ruled that the FNC upper receiver is the firearm. The letter also states the registered FNC sears can only be used with FN FNCs legally imported into the US. Sorry to be the wet blanket.
The upper was reclassified as the firearm by the ATF... but the sear is installed in the lower.
That makes the lower essentially an unregulated accessory because they didn't want to open up a can of worms with all the holes being drilled into FNC lowers to install the sear. Drilling holes for sears in receivers is typically verboten... look at AK's and AR's for example.
Since the lower is not the receiver (it's now just basically a large trigger pack for the registered sear), and ATF has no issue with drilling and milling on them, and they also have no issue with sears being swapped between other legacy rifle systems like H&K's or AR's from different manufacturers, we should be good to go. Hopefully...
SCAR owners wanted to install FNC sears into FN SCAR rifles with the hope that ATF would consider it a legacy rifle. Unfortunately, they subsequently ruled that it wasn't. The differences between a SCAR and an FNC are fairly obvious although they are somewhat similar in design on the inside. The Pindad is unquestionably a very close copy of FN's original FNC.
Given that the Pindad SS1-C is a factory built FNC rifle manufactured on FN licensed tooling, I think it has a good chance of being considered a legacy platform and sears will be able to be installed in these unregulated lowers. Bolt carriers will also be allowed to be welded up for trips since they are also an unregulated part.
The FNC design requires no modifications to the upper receiver/firearm to be used with a sear.
I think this will be the key factor in the ATF approving sear installations in Pindad imported FNC's... assuming they are allowed to eventually import them and decide to do so.
I'm keeping my fingers crossed!