"Most people who ask about modifying their radio are looking for ways to make the IC-746 transmit or receive additional non-amateur RF bands. The 746 already has general receive coverage from 0.5 to 60.0Mhs, so no "mod" there.
Modifying the IC-746 to transmit outside of the amateur bands is generally considered to be illegal. There are a few exceptions; like perhaps MARS/CAP, transverter usage, changes/additions to ham bands, etc..., but they can become controversial.
Icom will give you the correct mod for your IC-746 based upon your serial number, providing you give them a copy of your MARS license to prove eligibility. If you let Icom make the mod for you (small fee plus shipping), then the mod will not void your warranty
That said, MARS/CAP/TX/RX modifications for the IC-746 (and many other radios) can be found at:http://www.mods.dk/
Another source for the TX/RX mods is QRZ.COMhttp://www.qrz.com/download/mods-i-k/ic746.txt
Pay close attention, as there are two distinct mod procedures, which correspond to the differing placement of the programming diodes on the PCB."