Drain bulk caps as with any device.
If it's a 2 prong cord, replace with a 3 prong and ground the chassis properly.
You can use any PC or tun amp head cord...just cut the end off. Or install the modern recepticle for power cords used on all the other heads.
To gauge the wire size...whats the fuse size of the amp? That's dictates the current you can draw. Make sure the wire size can 'carry' 2xs the fuse size so the power cable doesn't burn up before the fuse blows.
Keep in mind your typical circuit breaker is only 15a.