XRs are usually bulletproof, I'm surprised it blew a head gasket. It sounds like you just need to check the basics, ie. valve adjustment, jetting (I'd clean the jets), float height, etc., spark plug. These things are usually hard to kill. If you are mechanically inclined get a manual, if not get a mechanic to look at it. I just drove out to Maine to look at my Dads John Deere and he did not have a manual, even though I'm a mechain it took me a lot longer to figure out why it was back-firing. It turned out to be the fuel shut off solenoid sticking intermitently. If he had a manual I would of been done in half the time and been able to perform a basic tune-up which I could not do caise I didn't have the specs. Good luck!