The \Magento\Sales\Api\OrderPaymentRepositoryInterface is implemented by the \Magento\Sales\Model\Order\Payment\Repository class

[dmitry_fedyuk (Dmitrii Fediuk)]

magento/magento2/blob/ffea3cd/app/code/Magento/Sales/Api/OrderPaymentRepositoryInterface.php#L16-L16

magento/magento2/blob/ffea3cd/app/code/Magento/Sales/etc/di.xml#L54-L54

https://github.com/magento/magento2/blob/ffea3cd/app/code/Magento/Sales/Model/Order/Payment/Repository.php#L18-L18