I think you have a few choices - but possibly none involve a 30mm bung hole if they aren't commericially available.
1. Fill the hole with suitable filler and re-drill at 25mm and use a "standard" bung.
2. Clean the wood on the transom up, seal with epoxy and tidy up the hole edges to give a nice smooth hole size then use something like this (35mm) to plug your new hole: 22mm / 40mm expanding Rib drain bung
3. Convert to elephant trunk with even bigger hole
4. Enlarge the hole and go for something a bit bigger like these: Zodiac Bombard Boat Drain Bung Plug Fitting Self Bailer Fitting Boating Sailing | eBay
55mm is a lot thicker than the average bung is, so you need to think about the other half of the hole. What I'd do is fill the hole with some fairly basic filler - poly filler would do. Just to get you a "solid" surface to work with. Then I'd pick the bung I wanted to use. Lets say you go with a 25mm one. I'd then dril out a hole maybe 35mm diameter (centre may be different to the old centre, but make sure you take out any cheap temporary filler. That should have sound dry transom on all is edges. Seal edges with neat epoxy. Fill the hole with epoxy + filler (baubles / fibres etc). Smooth perfectly flat and make look tidy. Drill that for 25mm. Fit 25mm on bed of sikkaflex. Bit without plug is now a waterproof tube.