Chris, I have been selling Yanmars since 1973, and the Co. always required dealers to fill out registration forms which includes model nos. and s/n. As Jared indicated the YM240 had a model 2TR20A engine, but there was a design change about 1975 when the headlights were mounted in the hood instead of on the sides, and the engine must have had a change of some sort (perhaps the air cleaner) because the engine model had a -X put on the end.
The Model YM122,12 H.P. single cyl. tractor had a MA10-N engine.
The YM146, 14 H.P. twin cyl. had a 2T75U-N, this later became the GT14.
The YM140(D) and 147(D) had a 2T75HA twin cyl. engine.
The YM135 had a 2T73A twin cyl.
The YM155 had a 2TR13A twin cyl.
The YM186 had a 3T72HA, three cyl.
The YM180 and 187 had a 3T72HA-N, three cyl.
The YM195 had a 2T84A, twin cyl.
The YM220 and 226 had a 3T75HA, three cyl.
The YM276 had a 3T84HA, three cyl.
The YM330 had a 3T84A, three cyl.
The YM336 had a 3T84A-N, three cyl,
Hope this helps, Gary J.