From looking at the CH manual, I don't see anything that would be too tough to re-wire, but it depends on what sort of options the engine you're going to swap has. Looks like the CH18 has a 5 pin connector with pins for starter, accessory/oil sentry, magneto ground, 12vdc and Carb solenoid.
The M18 has a 3 pin connector that has 12vdc (from engine charging system), oil sentry and magneto ground.
If the CH18 has a carb solenoid, you'll need to either find a 12vdc switched wire, or wire in a relay (or disable the solenoid).
The starter relay for the M18 is separate from the engine and mounted under the 1872 pedestal. If the CH18 you're installing has the built in starter relay (I think most do), you'll need to remove/bypass the M18 relay and either wire it into the harness or run the wire down to the starter relay.
Other than that, Magneto kill -- Magneto kill, 12vdc -- 12vdc and oil sentry -- oil sentry should get you pretty close.