The opening in that flange is small enough that you could slightly missalign the horn on the toilet with that opening and create a place for the waste to catch, collect, plug. Besides not being of legal size according to code.
If cutting out the neccessary fittings and replacing them from below is too big of a job, I would recommend you go buy-rent-borrow a 3" Ram Bit and use it to drill out the flange that is cemented into the 90* elbow and replace the flange. I prefer to use the PVC / ABS flanges with an adjustable stainless steel ring. Glue the new flange in and be sure to anchor it to the floor also before setting the toilet.
a 3" ram bit is probly about $40 - $50, hopefully you could just borrow one.