irtmks,
The fuse you use should be an easy fit, so any size from 3mm on down to 1/16'' will work fine in a 1/8'' vent (I've been using the skinny 1/16th fuse lately, I like it). Use real black powder if you can, it smells like rotten eggs when it burns, it makes a louder boom, and it makes beautiful white smoke. A 5/8-inch bore converts to .62 caliber, so use grandpa's old method for black powder rifles: One grain of powder for every caliber, so 62 gr. FFg to fire a projectile, (or just work your load up from 50gr. to what you like) and double the load (124 gr.) for a salute.
I've got the Dahlgren model which is the same bore size as yours, and it's a solid little cannon. Your gun looks clean, so check to see that the bore and vent are clear, and you should be good to go.
Send me a message with your ground address, and I'll send you a copy of the A & K pamphlet along with copies of some other papers that give their safety recomendations for their cannons. One of their dictums is to always use a safety rammer when loading the gun, (they call it a ram, and provide diagrams) and I think this is sound advice.