You cannot mix and match Webforms/MVC in a single request, so all renderings on a single item need to be either Webforms or MVC only.
The best solution is to migrate the solution to MVC, if you can't do that all at once then build out the site section by section to move to MVC, there is no issue having both a Webforms and MVC solution running side by side, just not in the same request.
You can override the out of the box behaviour using techniques such as Mixing Sublayout with Rendering – Webforms with MVC request or Using Sitecore MVC components on WebForms pages however these are not supported by Sitecore and everything may not work as expected, but it may be an option if there is no other possibility.