They are using instructions for the MT-12, which is the optional engine you could of gotten, but the price is 200 bucks more. you can use a short shaft engine, some require you to cut about 1/4 inch off the end so you can screw on the pilot shaft. Ignore the instructions about how to prepare the shaft that they talk about in the manual.
Some motors use a pilot shaft, some are threaded long enough that all you need is a clutch nut to hold everything down, then you just add a shim and the clutch bell, then top it off with a screw.
TeamXray RCAmerica Hudy Xlgraphicspro Paint/ Impulse-Rc Clutch shoes.
Xray XB 808 O.S. Speed Or Ninja JX