Whenever you get involved in matching trim on old houses it nearly always involves having cutting knives made. It's very rare to just buy a bit that looks similar enough to get by. You might find a cabinet shop or a furniture repair shop that has a shaper that can make a set of knives to make the trim. As simple as the design is, it might not cost too much for the tooling. If you have a table saw and a lot of patience you might consider grinding a cutting knife to go on a cutter similar to this one. There is a different cutter which only has one knife instead of three. With only one knife you have to run the wood really slow but it will work.