I've found a lot of times that you can pull apart clumps of the spaghetti into at least seperate source files, sometimes separate classes, without having to rewrite any actual logic. If you can steal some time to do that off of a bug fix or two, you will be way ahead of where you started.
Later, when more bugs come in, you might be able to modularize things a bit better. Slowly, you can get the spaghetti straightened out this way.
