I did the last one by drilling out the hole larger and then filling with resin with some shopped strand glass in it. Give it a little more strength than straight resin.
Like tpenfield stated, if the wood inside the motor mount/stringer is wet or rotten, you can't fix it this way. You'll need to evaluate the state of the wood up and down the stringers and across the transom before knowing how to proceed. You will end up ripping apart your engine coupler if the wood is rotten.
