You can ask as many questions as you like =)
When I’ve been making or updating payment plugins I usually copy another one such as PayPal and modify it to work with the API in the docs of the new payment provider, in this case Payza. These files are located in the payment folder.
I am actually in the process of rewriting the script as a WordPress plugin but if you have any contributions feel free to make a pull request on github. There is a dev branch where all the changes I’ve made since I took over the script have been going. I originally didn’t use github so the first commits contain a lot of changes.
I’m not sure what Duo is but I have added HTMLPurifier which will become obsolete in the WP version which will pretty much be a complete rewrite.