I have coded a few payment gateways earlier for WHMCS for an earlier company I worked with and am confident of completing this in less than an hour and will directly work on your server if you are okay with granting access.
Alternatively, you may provide a development install of WHMCS (free license if you purchased WHMCS directly instead of a reseller) and provide me access and I will code the same here.
Hope you will give me an opportunity to work on this project.
Thanks.