Before you do anything, make a chamber cast. This will tell you if it's a paper or thin brass case chamber. This will determine ball size, i.e. what will fit in the correct case.

If the ball is a few thou undersize for the bore, don't worry about it. 1 in 15 to 20 tin/lead will bump up with black powder.

That gun may very well be a light charge one, maybe 2 1/4 to 2 1/2 drams. Only shooting it will tell.

Make sure you wad it tight, and have adequate lubrication.