How to replace custom tags or expression with value from database in Dynamic Email Template

Hi I am getting an email template from frontend. This Email Templates looks like:

Hi ,
We have created an account for you. Here are your details:
Email:
Password:

and I am saving this email template in database. Later while any new user signs up I want to send above email template replacing the USER with username, with email of user and with password of user

I am struck at how can I find this tags or expression from above template and replace it with any dynamic values from db. Let say this values are USER:Test Email: test@test.com and Password: 123456

via Chebli Mohamed