Sizing 12V Automatic Circuit Breaker

I'm adding a second battery and a portable solar set up. Pretty pleased with the cables I made up and installed a master switch at the batteries. Here's the problem. The C/B mounted on the frame that the HOT side ties into is a solid block of rust. I got it off but it's not usable for the new cabling...really RUSTY! Easy to replace...right? Wrong. There's nothing at all on the exterior to indicate the AMP rating. The trailer has a 30AMP service rating. Is there a way to size this C/B based on the general electrical? The fuses in the trailer panel are marked 40AMP for the reverse polarity. Thanks