Could be worth checking the primary chain tensioner, they can make a wonderful knackered big-end sound if they are very loose.
I can't imagine anything serious being caused by ride along a gravel track.
The tappets are easy to check which is why I mentioned them first.
The best thing is if they are ok (ie can be spun but not lifted). That tends to imply the pushrod is straight and nothing horriffic has happened in the valvegear. On a standard bullet, it only requires you to undo one nut to check. (Actually, thinkng about it, best thing would be if one of the adjusters had come loose and you just need to tighten it again).
Primary chain tension is similarly easy. You take the primary cover oil filler cap off and you can reach in with your finger and feel the chain tension.
Sparkplug electrode can tell you a lot too. Sooty, burned, missing, covered in alloy filings or normal (phew!). If the timing had moved, that could cause a clattering noise as it predetonates, plug would probably be looking scorched/nibbled.
As I say, check simple things first. The last time mine went wrong it simply lost all compression and coasted to a halt. The first thing I checked was the tappet adjusters. The exhaust pushrod was tight and continued to be tight even when backed all the way off. This told me something serious was wrong. Then I had a look at the plug and the electrode was missing. At that point I called recovery. It turned out it had snapped a valve, the head of which had been hammered into then through the piston crown.
It's not unheard of for them to drop an exhaust valve seat either. That can cause a clattering noise, popping on the overrun and occasional temporary losses of compression (and surprisingly little other drama) but it's not the first thing I'd look for.
The reason it sounded like a rattle in the mudguard is because one of the mountings came loose! The ominous misfire on the way home was the fuel running low and needing reserve.
I fixed the mudguard and restarted it on reserve but one rattly sound from the engine was still there. I eventually noticed that the neutral finder was pointing at 3 - I thought I'd kicked it to N but it was in a false neutral. The ball bearing in a tin sound that I heard when I pulled up in my garage was the neutral finder mechanism rattling on the gears. Kicked it to neutral properly and the noise went away.
I guess I took 1 + 1 and made it equal 11. Just 3 separate issues which, up on that frozen heath, got concocted into something more serious.
On a plus note I know that my pushrods are correctly set and the plug is a nice nut brown and I now have a set of gaskets for the next "emergency"
Thanks all for their suggestions and pat yourself on the back if you suggested bodywork