Puddin' Man wrote:

, it -does- have a nasty crack in the gearbox housing all
along the fastener housing right by the drive shaft
(fastener(screw) is gone).

A garage door opener supply or repair company may have a spare gearbox
housing available, but cracks in that kind of plastic can usually be
fixed with epoxy, although it's far better if you reinforce any repair
with fiberglass or nylon cloth. Clean off all traces of oil and
grease, roughen up the plastic and use slow-cure epoxy because 5-minute
epoxy doesn't cure right unless it's mixed almost exactly right. Also
any places in the plastic that stick out and have screws going into
them should be reinforced with metal tubing (hobby shops and real
hardware stores sell brass and aluminum tubing) epoxied over them.