Hi Chewie,
I don't know the dimensions of that particular rifle but it will definitely fall in to the standard rifle class. dimensional restrictions for the standard class require that the forearm be no deeper than 2.25" from the center of the bore to the bottom of the forearm.
I also don't know if you have been shooting this game for awhile or not so if you have, please forgive the following advice. I like shooting open sights on ocassion as well. If I had an open sight rifle at the moment I would probably drag it to the range every now and again just to humiliate myself. I wouldn't, however, go out and buy an open sight rifle specifically for this game. There aren't enough matches where there is an open sights class and you will find it extremely challenging (impossible) to be competitive against scoped rifles. I am sure the rifle you have pointed out has a sight/scope dovetail so i'm not suggesting that you don't pick one up... just that you might want to give a scope a try.
dave imas