You can edit the config through the menu tools like menuconfig or xconfig or any other that does the job to tweak the kernel to only really have what your machine need. You also need to know well what hardware the machine have, for that:
will give all the info about it.
To edit the kernel takes time and one can't rush it, it isn't a trivial task, attention to every detail is necessary. If you don't know what one thing does research it, read about it. The advantages to compile the kernel is to gain performance, security, and other things, for old machines it does make a huge difference.