Yes, it can be done. The EBB is based on the UBW, which is a board that uses a PIC18F2455, which is in the same family as the 4550. So a lot of the existing code will work as-is with the 4550.
The one big piece of code that will not is the RC servo pulse generating code. That code (called S2 method in the source) uses the PPS peripheral which is not present in the older 4550 family of parts. So you would have to revert back to the older servo generating code. It’s actually still in the source, but is not used and hasn’t been maintained.
So the basic answer is yes, it can be done. You will need to re-create the basic EBB circuit – take a 4550, add USB and power sections, and two stepper motor drivers. Then you’ll need to modify the firmware a bit based upon which drivers you choose and what pins they are on, and the servo stuff as mentioned above. But it can be done with some work.