The B,C,D are ratings that relate to how strong the tire is. Typically the further down the alphabet you go the more pressure you can inflate the tire to. Creating a stiffer tire. Not exactly equal to load capacity but related. More a description of how well the tire can handle the given load capacity.
Understanding Tire Load Range - Town Fair Tire
If you scamp
has a 4 bolt rim I think 13 inch is the only one offered, at least commonly at tire stores. 14 and 15 inch rims will use up more wheel well space and make the condition of your torsion axle
rather critical. If the torsion suspension is getting a little old the trailer wheel well will sit low on the tire.
Typically changing tire rim to 14 or 15 inch involves replacing the axle
with one having more down angle to make it fit in the wheel well if the original was a 13 inch rim.
Scamps sit low as stock but that does help with wind drag when towing.