Better not pass on loads from the
tension of the rope against the capstan to the gearbox. *Got to send
those loads - got to be through some bearings - to a structural frame
in which the gearbox is mounted, subjecting itself only to torque.

A lot of gearboxes are used to run chain drives or v-belts, which can
have a substantial sideways load, so it might be just fine? I'd check
the manufacturer spec sheets--if you can find any--before I went ahead
and designed a jackshaft, frame, and lovejoy or double-row chain
coupling. Or maybe specsheets for several other similar gearboxes
from Boston Gear, Marvin, Eaton, etc. to see what the range of typical
values is.

But if you have to design something, they're pretty straightforward.
A chain coupling would probably hold up better than a lovejoy (3-piece
flexible coupling) under the higher torque loads on the output side.
SurplusCenter.com has both styles of couplings in the states, you may
find their pages useful for figuring out what you need before you try
to find them locally.

I'll also note that most of the similar gearboxes they have listed for
sale are rated only up to 1800rpm input. With a typical 3600rpm small
gas engine, you'll either need 2:1 input pulleys, or to find one with
a reducer box (some extend the camshaft out the side case for 2:1
auxiliary output, some come with a 6:1 ring gear setup, I'm not sure
if there is an add-on 2:1 gearbox available, but all the gearbox
versions I've seen are harder to find and more expensive--if you're
scrounging, 2:1 pulleys are easier and give you more cheap/available
engines to pick from).

Hope that helps,
--Glenn Lyford