I've done a quick sketch which is mainly from memory. The upper layout would be the preferred option. You may need a further T-piece in this connected to the battery to power the backbone cable and any sensors attached to it, though I think the power connection to the head unit should do it. If I remember correctly most head units have two power connections, one for the network cable and one for the display itself. The lower option should work from my understanding, but I can't guarantee it as I haven't tried it. MAybe try it first as it would obviously be cheaper to do.
Your Icom can only take NEMA 0183 so you won't be able to change this over to the new network wiring. Just leave that bit alone.
The EP-20 is relatively useful IMHO. It gives me fuel economy which is good, and once I set it up properly I should get oil pressure and engine load as well. I'm not sure what NMEA2000 the earlier Suzukis send out. I know the 04 engines were the first to have the data output.