Two Dimensional Lookup and Cell replace

I have seen several close but none that I think will help me...

I have a "Source/Target" worksheet that a user wants to update but the format can't be altered. The user will copy this data to an "Active" worksheet where they may delete rows and/or columns and update cells.

Once they are completed I would like to have a button they press that will loop accross the target worksheets column headers and find the proper column and 0 out all the cell values to a certain row "<End Here>". I would then loop down the header row and look for the matching value. Once i find this I would replace the 0 in the found Column/Row with the "Active" cell value.

I can use the function to find the cell address:
=ADDRESS(MATCH($A3;[Source.xls]Sheet1!$A:$A;0);MATCH(C$2;[Source.xls]Sheet1!$3:$3;0))

If anyone can help me with how to accomplish this I will be eternally grateful!

Thanks!