At last, a bit of luck.
The rear head is the easy one to get to.
Exhaust off, carbs off (leave the cables and coolant connecting pipes alone, just move them out of the way) cams out, undo small bolts inside the camchain tunnel first, then all of the head bolts.
You might need a long series socket for this, suggest a 1/2 or 3/8 drive at least, they're too tight for a 1/4 drive socket, it will most likely shear the drive end off.
Head off, fit new head gasket (all other reusable), fit replacement head, rebuild and retime, then you'll need to set the valve clearances, and probably re-shim.
Check three times before turning engine over carefully by hand.
But if you set the crankshaft in the correct place before you start it shouldn't have moved.
The in with the compression gauge, hit starter, observe healthy compression.
Refit exhaust, and ride it.
Then hide all your tools, burn the manual, and never, ever go near it again except to ride it, put oil and fuel in, and perhaps pump up the tyres.
![Laughing :lol:](./images/smilies/icon_lol.gif)
It's not falling off, it's an upgrade opportunity.