I was faced with this situation, only in that case it was a GE panel. I was stunned to find an absolute lack of information or assistance available from GE - or the distributor. Lot of help they were!
In the end, all I really had to do was walk through the warehouse with a tape measure. The second panel I opened had the exact correct guts ... making change-out a breeze. Now - Why was no one able to look that information up, to tell me that the interior of panel "X" was the same? Or, look up the factory part number, and simply sell me a replacement buss set?
Now, you may wonder why I didn't do a "complete panel replacement." First of all, the busses were damaged when the maintenance guy "cleaned' them with a Dremel - hot!
They were losing breaker connections to arcing one by one.
The commercial building had the panel set in a block wall, with something like 15 pipes coming into it. The most important point, though, was that this customer provided phone / internet services for hundreds of customers, and "going down" was a BIG deal.
Simply replacing the damaged busses with new ones - from a stock panel, perfect fit to box and cover - was a half-hour deal, with many circuits back up inside of 15 minutes.
Frankly, it concerns me that these are the same folks (GE) who make jet engines and nuclear power plants.