Can I use acrylic paint to touch up a guitar?

You could but I doubt you would ever be able to duplicate or blend the touch-up to match the paint that's already there. Clear polyurethane would be a good choice for a top clear coat. If you want a like new look, disemble the guitar and take the body to an auto body repair shop and have it spray painted professionally. You could leave it as is. A lot of people pay extra for factory new guitars that are given that "been on the road" look. Check out Fender guitars website. Some of their models look like they've been around for 30 years, scuff marks and all. Good luck with whatever you decide and rip off a few hot licks while you're thinking about it.