This a re-post of how I answered this question once before.
I’ve restored two Chestnuts with ends that looked just like yours. For one I bent new stems, for the other spliced in new stem ends. Then I had to splice in new inwale ends. I made the new stems and new stem splices a bit long, so I could cut them back, when I fit the inwale splices. A lot of cutting and nibbling away to get the three pieces to meet.
I’ve read that the stem is fitted into a mortise on the underside of the inwales. This also requires a lot of cutting and nibbling away and trial and error fitting. So I dumped that route. Instead, once all three pieces met and fit properly, I made a two piece brace for the joint, consisting of a small triangular piece, cut to fit underneath the deck and span both inwales, into which was mortised a five-six inch long backer dimensioned the same as the stem for the back side of the top of stem to fit to. Think of it as a wooden angle iron.
When you’re building on a form, it’s much easier to fit these four pieces—deck, stem, and inwales—together. The form is your clamp, your workbench. When this area is just a big hole, it really is, as Dick says, a difficult and weak joint. There’s not enough wood there to work with. Everything is coming to an end. So my thinking was that, with my limited woodworking skills, the best approach was to add some wood to the area to work with. And adding the wood makes a super tough joint.
