Come to think of it, the parts list for your 4 blade rotary does mention variations of the mower with three different engine brands. The Jacobsen 321, Briggs and Stratton engines, and Tecumseh engines. Each engine variant having its own model number ranges. So, yep, Tecumseh did make engines that Jacobsen used on their mowers. The parts list shows a parts break down for the 321 engine because that was Jacobsen's engine. I see that they say down on the bottom of the parts list that for parts for the Briggs and Stratton and Tecumseh engines you have to contact them.
Components like the carbs and ignitions could very well have been the same on the different brands of engines. I mean, the same Walbro carb might have been used by the two companies, but I wonder if Jacobsen would be able to tell you which Tecumseh engine used the same carb. I'm going to guess that finding out which ones were used on which engines would be something we would probably have to find out from the engine manufactures and cross reference ourselves.