Kenneth, Have D Motorgraders. You are going to spend time to change out the following if you get anything else except what is in your Model D.
262 (6cyl)diesel- need to move the front motor mount frame, possibly extent the frame where the radiator sits because the 6cyl is longer than the 4 cyl and rad needs to be moved too. Dont forget the flywheel and clutch assembly.
Combine (D-17 Z)engine- need to RR water pump, govenor, timing gear cover,and to align up the Hyd pump with the crank pulley you may need to cut the crankshaft. Need to do clutch and flywheel assembly. Well worth the trouble if you want a 63Hp engine instead of the 40-45Hp engine that may be in yours.
Look on the Carb side of the engine for a flat area with stamped ###"s like 45-##### or 17-##### this will tell you what engine you have. Hopefully you will have a 45 or D-17 engine (226) and not a WD (201)engine(identified by a bolt on plate on the side. Also the SN of the unit (located on the right side of the cab frame) tells you what sorta type of engine should be in the unit WD,45 or 17 etc..(looking in the Parts Manual)
Had a 45 engine on mine and since I do not have a garage to work on it, if I had to do it all over again I would just purchase a rebuild kit with all seals, bearings, sleeves, rings etc.. and have the head/valves done. The engine was a good price $400. but the shipping cost me about another $500 after all was said and done. then the labor to do it and the time. For the cost it would of been quicker,cheaper and less headaches to have the engine rebuilt and I would of known it was an exact fit. Am still working on it cause it is taking time.
Great machines and dont get discouraged, keep on trucking. Worth the labor and they do a great job. Call me if you need more info 505-470-4638