Dear Pedro,
In this case, you should contact the payment module author to make compatible that module with WHMCS 8.1.x. Otherwise, you will fall into a big security problem with your system or something else. Because breaking an old system security is easy.
However, please send a message using our Envato Market (themeforest) profile contact form to click here https://themeforest.net/user/themelooks#contact (screenshot as reference https://prnt.sc/10o0iwt ) to get the older version of Bluishost WHMCS Template.
Best Regards,
ThemeLooks Support Team