I have created a HTML portlet with iframe and configured action link in cost plan detail page to navigate to respective financials page in new UX. for the benefit of other users, I am updating the portlet code I used.
<html>
<style type="text/css">
html, body, div, iframe { margin:0; padding:0;}
iframe { display:block; width:100%; border:none; }
</style>
<body>
<script>
var url1 = window.location.href;
var location1 = url1.indexOf("id=");
var projectKey_1 = url1.substring(location1 + 3, location1 + 10)
url1 = url1.substring(0, url1.indexOf("/niku/nu")) + "/pm/#/project/" + projectKey_1 + "/financials";
var myiframe = document.getElementById('myiframe');
myiframe.src = url1;
</script>
<iframe id="myiframe" frameborder="5" height="540"></iframe>
</body>
</html>