Template Migration Best Practice

Greetings,

We're migrating from 3.7SP4 to 5.0.
So far so good. However, the migrated templates (masterpages) don't read the default skins of RAD controls that are used on the page.
For example, we use a RadTabStrip as top navigation menu. In our 3.7 theme (under ~/App_Themes) we have a:
- RadTabStrip.skin (a minimal RadTabStrip tag that calls its own "skin")
- a TabStrip folder that includes images and a style sheet which corresponds to the "skin" defined in the RadTabStrip.skin above.

This logic works in version 3.7 but has not migrated well.
I'd like to ask what would be the best practice to deal with such situations:
Do we need to create a widget from scratch and use it to replace our old navigation menu, or is there a way to still use the old logic?

All other theme styles work well in migrated templates.

Thanks in advance.

Additional issues migrating from 3.7 to 5.0:
1. Images called from the file system  (and not from Sitefinity Libraries) do not display, even though they are there.
2. Global resources saved under ~/App_GlobalResources cannot be read by custom user controls; the resources are there but are obviously not seen by Sitefinity.
3. Custom user controls created for User Registration, Managing User Profiles, Password Recovery, etc. (both for front-end and back-end) require complete re-writing.

First issue is the most problematic. It arises on our development environment (Windows 7, IIS 7.5) when using the Visual Studio default server (without manually mapping the new project in IIS).

So, here are some tips given by Sitefinity techs regarding images and GlobalResources not accessible after migration:
1. Images stored in a folder called "Images" are not shown because the folder name is reserved by default by Sitefinity for image libraries. Follow this link to resolve the problem: