The largest 2 stroke available is now the KTM 300MXC. It would be easy to convert one to an SX. The SX tank and shrouds is the easy part. The tranny gear ratios are also different. Unfortunately the old 380SX is a much faster bike than a 300.
I think I read or heard somewhere that KTM could make a custom bike upon request, using the 380 motor and current chassis from the 250SX? I'll ask them & let you know ....