there is only one CORE file that has been modified .. html_output.php , you can identify all patch files and remove them , and replace html_output.php either with new file , or simply remove the extra code that was added to this file .. you can find that information in read me section of patch .

before doing so , download and install Debug utility , follow instruction on how to use it . this tool will generate a error log , pointing at problem file or files that causing blank page. post back the error if you can not figure how to fix it . https://www.zen-cart.com/tutorials/index.php?article=82