HELLO FELLOWS and possibly a lady or two.

I just bought a new head unit and some door, rear and tweeter speakers. When I pulled the existing head unit (installed by previous owner) and dicovered some problems:

The original Honda/Acura harness was replaceed with some cheap, small harness and the wiring for it is way hacked up.

The wiring to my speakers is brown, thin and crumbly.

I guess my question is, can I replace all the speaker and head unit wiring in my car? Is this a regular problem? And what size speakers can you fit in the stock locations?