I converted from an ammeter to a volt meter and also installed a high output 1 wire alternator on my ERA. I was also concerned with that wire being unfused so I simply added a fuse. You can get a 125A fuse from Littlefuse that works well (part no. MEG125BP). I simply "bolted" it directly to the solenoid terminal and then shrink wrapped it to protect it. I've attached a picture.
