You can attach a Mono-Sight to any Heavy weapon, including the ones with Unwieldy.
However, since shooting an Unwieldy weapon normally takes two actions (and you only get 2 actions per activation), you won't be able to Aim before attacking with the weapon, meaning you won't benefit from the Mono-Sight under normal circumstances.
This being said, some wargear/skills/etc. let you shoot Unwieldy ranged weapons as a Basic action (the most common being the Suspensors accessory, or Third arm for genestealer cultists). If you also use one of those, you will be able to Aim and Shoot your unwieldy weapon and get the Mono-Sight bonus.
Depending on how your group interprets the Aim action, it may also be possible to Aim as the last action of one activation and Shoot (double) during the next one if the fighter activates twice in a row (thanks to Overseer shenanigans for instance).
Then Third arm is not an option to you but you can still get Suspensors as a rare trade rom the Trading post.
Also note (since Cawdor have easy access to Brawn skills) that Bulging Biceps won't help you, as it only applies to close-combat weapons (though having it work as Third arm for ranged weapons is a popular house-rule).
Of course, don't try to attach a Mono-sight to your Heavy flamer, as it does nothing for Template weapons (and I would advise against taking a Heavy flamer in the first place, it's sadly overpriced). But for Heavy crossbow/stubber (and other heavy weapons from the trading post), the Mono-sight + Suspensors combo will work fine.