Implement new factory class for instantiating a payment object throughout core

OLD

$paymentProcessor =CRM_Financial_BAO_PaymentProcessor::getPayment($payment_processor_id, 'live');
$paymentObject = CRM_Core_Payment::singleton('test', $paymentProcessor);

NEW
Preferred -
Civi\Payment\System::singleton()->getById($paymentProcessorID)

or if you already have the array call getByProcessor - (getByID actually does this call internally)

Civi\Payment\System::singleton()->getByProcessor($paymentProcessor)

You can resort to getByName if you REALLY have to - but I can't think of a case where you should have the payment processor name and NOT the ID