Easiest is to just reproduce the Pix configuration with equivalent functions in ASA 9.x syntax.
You should first analyze the Pix configuration to detect and remove any unused configuration lines or stanzas.
90% of most firewall configurations are the object, access control lists and NAT rules. These are pretty easily migrated manually.
While there are migration tools, they are imperfect and your Pix is far far past end of life so you would likely have to perform a difficult and time-consuming upgrade before getting to a configuration that's even usable with the migration tools
