Like Greg said, the ABS hull is rather flimsy and too much solvent will reek havoc on the material.

You can however make an inside doubler for the transom out of styrene (ABS) sheet and glue it in place by running a tiny(!) amount of Acetone or MEK in the Swiss cheese from the outside, while holding the doubler in place, the ABS will weld solid after the solvent has evaporated. Make sure there are no burrs between the transom and the doubler before applying the solvent.

I use a goo made of ABS scrap pieces, solved in either one of the solvents above in an airtight container (35 mm filmcan works nice) and fill the holes with that.
You can make the goo as thick as required.
Again; don't overdo it on the solvent!

I've built entire ABS sheet hulls from scratch with this goo and it works very nicely.

Regards, Jan.