Guidelines to change or add a payment engine

[Arsenic Aeternus]

My question is how difficult  is to add Khipu payment to catarse, if you have some sort of guideline I would totally love that really, I was trying to add that payment who has its own gem available, and it's working standalone in a simple webpage for testing usage.
I can't really figure out how to manage to replace one of the payment engines who are implemented inside catarse as catarse pagarme for example. Variables don't match so I can't just replace in a "cheap" way the code .

That's the thing. As I said I would totally appreciate some kind of help, I really like this project but this limitation has given me some headaches...

[Jevgenijs Сernihoviсs]

Some basic guidelines would be really helpful. I'd like to pay someone to develop Yandex.Money payment gateway for catarse.