I thought that Mack built their own engines. I used to like the old FDNY Cherry Pickers from the late 70's. The one in my neighborhood had a manual transmission.
RE:School buses. School buses are built on a standard truck chassis and they usually have the same diesel or gasoline engine. A couple of days ago I spotted two International vehicles side by side and they were identical at the front, but one was a school bus and the other was a truck.
Most vehicle manufacturers build their own engines. I can get complicated with cars because they use many different engines. Very often one car is available with engines of different displacements, cylinders or type of fuel. Example: In the U.S. a 1999 BMW 5 series sedan is available in two flavors, but in Europe the same car is available with several engines from a 2.0 liter 4-cylinder to a 4.4 liter V8 and a 3.4 liter 6-cylinder turbo diesel. Like buses, I can tell the number of cylinders by it's sound.
Wayne