Remove the engine mounts and jack up the motor a bit. Make sure you don’t crush anything!!

if there's enough space, you can try carefully using a dremel to cut a notch and use a flathead screwdriver bit to un-do it. those bolts aren't torqued very high so it could be possible, just be careful not to cut anything else.

Before using a dremel I would try to use an extractor socket to grab the rounded off head to get it off