The site linked below is one good source for V-belts. Forget about part numbers. All you need is the length and width of the belt. I like that this supplier gives the outside length of each belt, as opposed to the 'pitch length' that some suppliers use. I use a length of masking tape to measure the length around the top flanges of the sheaves. The proper width belt will ride flush with the tops of the sheaves.The width of the belt (A, B, C, etc.) is just a measurement taken between the flanges at the outer circumference. My Atlas 10' lathe uses two A section belts.
If you suspect that the current belt is an incorrect replacement, you'll need to study the travel distance of the belt tensioner to figure out the best length for the new belt.
The power twist style belts work. They are convenient to install in difficult places, and the Atlas lathe is a difficult place because you'll need to pull the lathe spindle out to install a new secondary belt. That said, I'd bite the bullet now and install a good quality V-belt. It should last a long, long time. Mine has been going for over 40 years.
Hope this helps.
http://www.totallybelted.com/V-Belts..ates-p677.html