The 6A12 is a 200hp 2 liter V6 found in the FTO.

Supposedly the 4G63 can use the 4G32 transmission, so a 4G63 from the GTi can fit the Sigma... and supposedly, again, the 6A12 can share FWD trannies with the 4G63... so it might be possible to use the original tranny with that engine... or not... it'll probably be hell to work out the wiring.