If I follow, you want to "fuzz" your PIX to test for isakmp vulnerabilities. If so, you may be interested in the protos isakmp fuzzer.
http://www.ee.oulu.fi/research/ouspg...ng/c09/isakmp/

If you are referring the the vulnerability I think you are, its not a problem isolated to Cisco devices, but most IKE implementations are vulnerable to DOS attacks. You are also not vulnerable unless you have enabled IKE and if I remember correctly, PIXs (and routers for that matter) have IKE disabled by default.