Any scope will work but there is an advantage to one with a balistic reticle as the trajectory is fairly radical.  I have a Nikon Omega on mine and they make a new model:

I have a couple of suggestions to make this less painful.  Use Barnes sabbots preferably the 250 grain with the blue plastic tip but any of them will shoot a lot better than the competitions bullets.  Powder charges in modern rifles can go from  80 grains to 150 grains of powder.  If you use pre formed pellets  two pellets is 100 grains and three pellets is 150 grains.  I have one rifle that will group well with 150 grains but all of the other muskets I have shot did much better with 120 grains of loose Tripple 7  FFG powder.  I would try some at 100  and some at 120 and see what groups best. It is VERY important to run a wet patch then a dry patch down the bore after every shot to maintain a consistent seating depth - failure to do this will result in a pattern that looks like buckshot.   Sooner or later you will run a dry patch down the bore and it will stick there and not come out - simply pour a little powder solvent down the bore and once the patch is wet it will come right out.   Never forget to put grease on the threads of the breach plug or it will ruin the rifle.  I find that muskets shoot better after some use than they do brand new.