I would say you can patch the floor. I would get rid of all the loose concrete and chunks that are small and cracked to avoid a difference in expansion and contraction of old and new. So, make sure what is left is solid. Make a paste out of Portland cement and sand and paint all the points of the old concrete that the new concrete will touch. This will help the bonding. You can use a concrete mix or just Portland cement and sand to fill the void. If you use only Portland and sand you will need to get some pieces of # 10 wire mesh (used in sidewalks) or expanded wire lath for reinforcement. Also, drill some holes in the edges of the old concrete (1/2" diameter x 1" deep and put some pieces of 1/2" reinforcement rods in the holes and have them stick out enough to be in the new concrete. You can also use the reinforcement and wire with the concrete mix also. Good Luck.

Calcats ;)