How to modify port source code before 'make' ?

[Adrian]

Hi,

I'm trying to install a port.. I know a little and can
make the changes I need to but when I 'make'  it
refetches the file from /usr/ports/distfiles
and scraps the changes I've made!

I've tried updating the dist file and putting it back in
that directory but it seems to perform a hash check
on it!

Is there a standard way to do this?

Thanks,
Adrian.