AM, I would use a "normal" twist drill to size the holes for the dowel. I think it will give you a more accurate hole size. Also, what is the airplane you are now asking questions about? It seems you have brought a fourth different model into the mix without telling us. That's how the confusion over what type of engine mount it was that needed to be redone occurred. I'am going to guess that this one is a Goldberg Eagle 63 trainer. No matter, for general background information, this type of wooden beam engine mount was a very common "old school" method of engine installation going back to when all models were kit built, ARFs didn't exist yet. Some kits and even some ARFs still use this type mount, but the separate fiberglass/nylon type is much more common today. Both work well. IMHO, in this case it will be less work to dowel repair this mount than to cut off the cheeks and beam mounts and then install the newer type. A fiberglass/nylon mount will require removing the fuel tank (you probably should anyway) and installing blind nuts on the backside of the firewall for the mounting machine screws. Not a big deal as you have a removable fuel tank access hatch. You also loose the look of the side cowl cheeks if that matters to you. Pick your "poison"!