That mostly agrees with my experience. I think the vapor degreasing is
probably unnecessary; regular solvent cleaning should do. To
successfully weld or braze a complex cast iron gear housing with minimal
distortion requires enough preheat to burn off all traces of oil in the
pores of the casting; this happens at a lower temperature than is
required for distortion control and additional crack prevention.
I did over a foot of cracks in a bulldozer transmission casing a couple
of decades ago by building a preheat oven on the floor consisting of a
layer of insulating brick from a kiln supply and a top box of 1" ceramic
fiber insulating board cut to size with a hand saw and tied together with
bailing wire. Propped casing on insul bricks, thermocouple bolted to one
corner, propane burner under, preheat to ~800 F IIRC, turn off burner,
remove box lid and weld till temp drops to 600 F or so then reheat.
Double gloves and heavy shirts advised. I used 55% Ni stick, which is
somewhat difficult, practice on scrap if you want to use it.
For your part I think that brazing might be better. I would try
preforming brazing rods or better sheet if you can get it to fit the
joint, possibly hammer rod flat, preheat with broken piece in place,
remove cover of oven, lift out broken pieces and brush flux on both
surfaces, insert flux covered preforms, heat with torch until preforms
fully melt and broken part can be pushed down to fully seated and cool
slowly. This requires more heat input than arc welding into ground out
groves on both sides of the crack, so it may require more preheat to
prevent distortion.
A search on "cast iron welding preheat temperature" or similar should
turn up a lot of information on how to select the best preheat
temperature for the job, likewise I would do a search to find the best
brazing alloy for filling a thin crack in CI; you want something that
will flow well.