Customizing an install

This is a very rudimentary question, but so is my understanding of  
how this building process works.

I wish to change a few parameters for a game (amphetamine)  I found  
the file in the source that holds the paramters, and I know what I  
want to change them to, but I can't figure out how to change them and  
rebuild.

When port builds something, it always uses the bz2 file, right?  How  
do I tell it I want to specify a different source for one file?

Or, how do I use the portfile to build independently of the port  
command.

Or, how do  I do this in a more sensible way than I can come up with?