It's no different than a ski boat conversion from OMC to Mercruiser, the transom work and motor mounts have to be retrofitted to support it.
I believe OMC Cobra conversions are easier than the OMC Stringer outdrive.
If you're keeping the engine from the OMC, I hope you have a GM inline 4 cylinder. I'm fairly sure the Ford 4 cylinder is a no go.
Don't forget, you'll need ALL the mercruiser components. (mercruiser engine mounts, manifold, riser, shift cables, mercruiser control box, trim pump, etc.)