The Mugen X12 is not based on the RRL Novarossi engines. It is an NS12 based engine.
It is referred to as a long stroke engine because the bore (piston/sleeve diameter) is less than the stroke, in this case 13.80 mm x 14.04 mm
The RR12L engines have a 13.70 mm bore x 14.25 mm stroke, which makes this engine a longer stroke engine. As far as I know, there are no other engines like this model, except for the yet to be released new Novarossi N12LL models.

These RR12L3 or RR12L5 engines are awesome on a short track and have loads of low end torque and have been extremely reliable for me.
Another awesome engine to give consideration to is the Collari LC12 by Werks racing. This thing winds and pulls like nothing else I've tried and uses AAC construction to boot.