As it is a press-fit pinion, it does take some careful alignment, support of the backshaft, and use of an arbor press. I did it on both of our RedLines and 57 Sports we ordered, almost wrecking the first one because I didn't have the backshaft supported well enough and the pinion looked like it was on crooked but all worked out. I ended up using the side of a nut to press directly on the backshaft, with the pinion resting on the base of the arbor press.
It made me long for my days at AndyMark using the rudimentary prototype of their new pinion press tool
they made out of gear stock, and 1293 promptly ordered one because one trashed RedLine practically pays for it. (Seriously, buy one if you see small-can motors in your future.)