Challenger Bus panel
Haven't heard back from you, so here goes. 

About the panel/breakers

This thing you have is a meter-main with 12 breaker spaces.  The main is "backfed" meaning the main breaker is just one of the breakers.  

The Challenger breakers are known to be defective. And because of that, many electricians treat Challenger panels as if they're defective.  They're not. There's nothing wrong with Challenger panels.  The breaker problem can be very easily solved: Eaton "BR" breakers are cross-listed for it.  Take a look at any BR breaker, it says 


  Type BR
  Type C


Type C is for your panel.   Ba da bing! That's easy.  

By the way, if the home inspection report had said something about that and you said "meh, whatever", replacing those Challengers with BRs would have prevented this.  So next time you see something like that on an inspection report... 

About that bus

Your plan, as you have presented it, is to "just replace the bus" using a components supply chain that does not exist in real life.  This brand has been out of production for 30 years.  Compatible buses go in the trash every week, but nobody thinks to upcycle them onto eBay or Craigslist becuase they are obsolete and connecting with a buyer is extremely unlikely.  There are companies that specialize in old Challenger panels, and you can try, but this is an odd duck even among Challenger. 

So replacing this bus is not gonna happen.  

As far as finding a modern bus that will fit, the problem is panels of this style are also obsolete: the very compact buses and gutters are now illegal due to "new" rules regarding wire bending radius (see that neutral wire for why). 

So fuggedaboutit.  You only have 2 options: Save this bus or replace the entire meter-main assembly.  Chassis and all.   I'll talk about saving this bus. 

Saving the bus

The bus position containing the melted breaker (9-12) is permanently useless.  So now we have two questions: 


Is the bus underneath the fried breaker intact enough to use the bus area below it?  (Because the feeder wires will reach there). 
Is the bus underneath the main breaker intact enough to continue in service? 


Either way, we are going to change all the Challenger breakers here for Eaton BR/C.  The Eaton BR2100 ought to do the trick, at $40 each.  Betcha didn't think you could get out of this for $100, did you? :) 

If all busing is in good shape

In this case, we leave the main breaker at its position, and fit the feeder breaker in positions 13-16.  The cable will reach.  Easy peasy!  

If the bus below space 9-12 is fried, but the main is OK

In this case, we leave the main breaker where it is.   We move the feeder breaker to positions 5-8.  Any remaining breakers go to positions 1-4; no breakers exist below the main.   This will involve extending the feeder wires; pull new wires if they're short, otherwise extend them with insulated Polaris lug splices. 

If the bus under the main is roached

In this case, you will need to remove the bus bars which come off the meter socket, and replace them with #3Cu XHHW wire.  This will need to be physically protected where they come through the hole.    

The main breaker is then moved to spaces 13-16 (below the melted breaker), and it will need to be bolted down the same way it's bolted down now.   The feeder breaker is moved to spaces 17-20.  That leaves spaces 21-22 for anything that is now above the main breaker. Remember this panel permits "BR/C" duplex or quadplex breakers, which you can put in spaces 21-24.