I can't see why most riflescopes wouldn't work as I have seen the omega packages here with Bushnell riflescope (which measure around 12" in length). So you should be able to buy any scope and mount it with the appropriate mounting hardware. If you want a BP/Shotgun scope the difference is generally overall length, power and Parallex set at 50 vs 100 yards. Personally I use the Bushnell Elite (rifle scopes) series on my ML and had no problems.

I may be wrong but I can't see why you couldn't use almost any scope with this ML.