The easiest way to know the right replacement for your drive belt is to read the codes and tell them to specialists which will return the right item for you to purchase.
When you can't read the code you can measure the belt.
When the belt is missing you need to get a cooking twine or similar
Here we can see the standard setup of a bike. As you can see there's a tension controller to keep the belt tensioned in the right way
You should make a round around the big pulley and the small one (green) with your cooking twine and so you will know the size required. Send this information to a specialist to know the right model.
Discover at the following links more information about drive belts
Exercise Bike drive belts https://www.partsfortreadmill.com/stationary-bike-parts/bike-transmission-belt/
Treadmill drive belts: https://www.partsfortreadmill.com/parts-for-treadmill/treadmill-drive-belt/
Multi groove drive belts for gym and high end home use equipment
https://www.partsfortreadmill.com/commercial-equipment/treadmill-drive-belts/
https://www.partsfortreadmill.com/commercial-equipment/treadmill-drive-belts-j10/