the pliers are just an easier way to cut and punch a hole in the leather belt. If your belt is too short, then buy a new belt and start over. They come long so you cut them to the length you need. They also come in a few diameters - but most of the differences are for what you prefer, your treadle won't care. They sell for $5 - $10 and are readily available.
I consider "vintage" to be non - electronic, which is about pre 1980 machines. My significant interest is in machined from around 1900-1930.