You need to FILE-COMPARE the files of the TEMPLATE, with their corresponding CORE FILES.

So...

1. Unzip the template you are keen on, onto your hard drive.
2. All of the files in the template will (or ought to) have a CORRESPONDING file in the default zencart software.
3. Use a program like WINMERGE to compare the file in the custom template, with its counterpart file in zencart 1.3.8a

So if the 1.3.5 custom template file is:

includes/templates/CUSTOM-TEMPLATE/templates/tpl_product_info_display.php

You need to use WINMERGE to compare it with the 1.3.8a file it corresponds to:

includes/templates/template_default/templates/tpl_product_info_display.php

Winmerge will highlight all differences between the older 1.3.5 file and the 1.3.8a file.

Then, you make sure that code differences that show in the 1.3.8a file are "inserted" into the 1.3.5 file. This may (or may not) require that you "overwrite" the old/redundant code in the 1.3.5 file - you should have a basic knowledge of php in order to judge whether the "new" code is relevant.

When you've compared and edited all the 1.3.5 files in the custom template, the module should work.