There are a few different basic universal numbering systems for V-belts here in the US. Some belts carry more than one number because that given belt meets more than one standard. Other belts just carry one number, even if they may fit another application that is specified by another standard. Some belts carry a manufacturer's stock number as well as a universal specification number.
A half inch wide belt would be 4L in automotive nomenclature or an A series in industrial nomenclature. The numbers after that are the length. 450 would usually mean 45". If you can't get a 4L450, then you might look for an A43. If an auto parts store can't look up a belt by those numbers, then try an industrial supplier. The automotive guys may make a 44" belt & a 46" belt without making a 45" belt, but the industrial guys may fill in that gap. There are also metric measuring systems in existence. You can change to a belt of a different length by changing the diameter of the existing pulley(s) or adding an idler.
You need to start by being sure that you have the correct width of belt. The belt should sit high in the pulley & never touch the bottom of the pulley groove. Most V-belts have a 40 degree angle, while the pulleys they ride in have around a 32 degree angle (actually 30-38 depending on the standard). The mismatch is there on purpose. The pulley grabs the belt by squeezing the sides. If the belt has worn to the point where the center of the belt touches the bottom of the pulley, then the belt is past it's service life & will not grab properly, no mater what you do. If the belt is the wrong width for the pulley, you are out of luck before you even start. 3L means 3/8" wide. 4L means 4/8" wide (4/8" = 1/2"). 5L means 5/8" wide.
The belt length is measured on the outside circumference in the automotive measuring system. The industrial system often measures the inside diameter. The difference is usually about 2". A 450 automotive length is about the same as an industrial 43 length.
This is a pretty good basic primer on belt measurement -
https://www.regalpts.com/PowerTransmissionSolutions/Other/Belt Drive Monthly's/Form_9806E.pdf
If your belt is making dust & slipping, then either it doesn't fit right or it isn't aligned correctly, or it is not rated for enough horsepower. If the HP rating is not enough, you can either go to duel belts, or increase the diameters of both pulleys by the same ratio & get a longer belt. You can also retrofit to a timing belt with timing belt pulleys. Those don't slip at all.
If you want to go all the way down the rabbit hole & read up on all the specifications that apply to V-belts, then you may want to look here (assuming that you have enough time, interest & coffee) -
https://www.baldor.com/mvc/DownloadCenter/Files/MS4050
http://www.bearingsandindustrialsupply.com/pdf/heavy_duty_vbelt_drive.pdf
An example of a belt with more than one numbering system listed -
https://www.vbeltsupply.com/a43-classic-belt.html