The topper hs no scope mounting provisions. It is simply a brass bead sight
Many, many hunters started (and some still do) hunting with shotguns exactly like the Topper. The Topper is a very well made shotgun
especially for the money. I have one myself.
Certainly newer, rifled barrel shotguns with scopes are going to outperform something like a Topper, but as with any equipment, you just have to know the limitations and operate within them to get satisfactory results.
I took my fair share of deer as a young grasshopper with a single-shot myself. I remember shooting my first deer from behind a huge deadfall in a meadow down below the apple orchard. Sweet memories.
My suggestion would be to grab say 3 boxes of basic "pumpkinball" rifled slugs from Remington (Slugger), Winchester (Super X), Federal (Power-Shok), Federal (Tru-ball) and Brenneke (KO).
Carefully shoot some of each make and see which works best for you and your Topper. Try shooting say 25 and 50 yards. (Go 75 if things work out well enough for you to do it) Spread the shots out so the barrel doesn't get all heated up, this will affect your results. Some usually work better than others. And be sure to clean the barrel and allow it to fully cool between different brands so you start them all on an even playing field.
Most of these slugs can be had for about $4/box so your outlay is like $60.00.
I wish you well in your endevours and welcome to the most wonderful of playgrounds God has ever given us - the great outdoors!