Two years! That's how long I've been putting off this project! I've got some time off from school this semester, and this is on my list of projects to complete, so here I am.
I was originally going to just get a new pickguard and some strat pickups. After looking at my options, I bought a guard from guitarpartsresource.com. They were the only site to explicitly state that their guards are for USA mustangs, and no one else responded to my emails.
I was going to get a white pearl, but after seeing the pic in this thread:http://www.jag-stang.com/forum/viewtopic.php?f=3&t=17641, I realized that a black pickguard on a white body looks pretty bad-ass, in a kind of utilitarian lets-get-down-to-business kind of way. Plus, it didn't hurt that black was the cheapest material at $22.
I was pleasantly surprised at how well it fits. The original cellulite guard had obviously shrunk over time, but I'm not sure if Fender pickguards are ever completely flush against both the bridge and control plate. The allparts pickguard fits very snugly, and while the screw holes don't line up perfectly, they're not so off that I'll have to drill new ones in the body. I might have to route the neck cavity a tiny bit. I wish I had snapped a picture so you guys could see.
Anyway, both strap button screws were so loose that wearing the guitar standing up would be a concern, so I plugged them each with a toothpick and some sawdust mixed with glue, then re-drilled them. After doing that, I took another look at the ugly, chiseled routes and wondered if I could plug those, too.

So I began to!

I'm not anywhere near being done, and I don't know much about woodworking, so it's been slow going. I plan to work on it sporadically over the next week or two. The filler wood is 3/4" poplar from the shelving section of Home Depot. You can't tell very well from the photo, but that's slightly over flush with the surface.
I'm going to use sawdust and glue to make a paste to fill in the gaps,then sand the entire surface under the pickguard nice and flat, maybe apply some sort of sealer or primer, and finally route the original, correct pickup routes. I guess if I wanted to do a complete restore (as well as completely hide the repair), I could refinish it, but decided I neither have the expertise nor the will power. Refinishing a guitar this old and cool just seems plain wrong.
So that's the story up to here. I'll upload pics as I slowly continue.