Hello,
The "banktransfer_end.php" file isn't supposed to be empty, and I think that it's the appropriate file to edit in your case. Did you try to add some text in it and tested it ?
Here is the content of the "banktransfer_end.php" file :
<div class="hikashop_banktransfer_end" id="hikashop_banktransfer_end">
<span class="hikashop_banktransfer_end_message" id="hikashop_banktransfer_end_message">
<?php echo JText::_('ORDER_IS_COMPLETE').'<br/>'.
JText::sprintf('PLEASE_TRANSFERT_MONEY',$this->amount).'<br/>'.
$this->information.'<br/>'.
JText::sprintf('INCLUDE_ORDER_NUMBER_TO_TRANSFER',$this->order_number).'<br/>'.
JText::_('THANK_YOU_FOR_PURCHASE');?>
</span>
</div>
<?php
if(!empty($this->return_url)) {
$doc = JFactory::getDocument();
$doc->addScriptDeclaration("window.hikashop.ready(function(){window.location='".$this->return_url."'});");
}