The PIX isn't really incorrectly translation the packets. If you're using PAT, IPSec will fail because IPSec uses ESP and AH which are not TCP or UDP (IP ports 50 and 51, I believe).
All you should need to do to make it work is to make sure that the client has IPSec through NAT enabled and that the concentrator has IPSec over UDP or TCP enabled. In the negotiating process, they will select this method of transport.
Hope this helps.