I'm still yet to do mine, but I've picked up a shallow mount pioneer sub from trademe (120w nom by memory) that should slot in. haven't tried on the original amp for now, as i'll give it a better source later. Andy above using a Hi-Lo converter is an easy solution! Some older amps have high level inputs too.
I'm going to strip the rest, as confirmed the front speaker woofers have lost their foam surround and don't want to dabble with fixing/replacing those drivers. At the recent auto sound&security, they had a single dim headunit mounted into the McIntosh headunit area quite nicely so I'm going to try this.