I have the same bike; mine had 12,600 when I bought it, and after riding it since April, it now has nearly 16,000 on it.
One often overlooked problem with these engines is the intake/carburetor mount; these develop cracks that may or may not cause the engine to run poorly, but in all cases, will lean out the mixture enough to eventually burn the intake (and/or exhaust) valve. A burned valve face will be severely discolored, and will be chipped around the edges, causing the compression loss and eventual engine failure. IF enough of the valve face chips off, it can cause scoring in the cylinder wall, or crack the piston.
If you're lucky, the above has not happened (mine made it to 12,600 with a cracked intake), and you simply have a blown head gasket.
Engine removal and installation can take (no joke) 30 minutes if you know what you're doing. I installed my engine by myself in my living room in the middle of January!