Assuming that it's not a Williams, Taito, or one of the many other cabinets that had dados that the bottom fit into. If your going stock replacement on one of these, the floor is bigger in dimension than the inside dimensions of the cabinet.
Okay, then, jigsaw the piece out, chisel the leftovers, then route out the dado slot...
OH, rain
BOWS. You would have to slightly disassemble the cab in order to put the new piece into the dado slots.
This does beg the question... if you have to go to this level of repair on a cab, does using the exact original method matter? Sure, maybe the original method would require disassembly, but couldn't you do the repair by gluing/screwing from the outside/support pieces underneath?