I think that sparks must have found some old cleaning rags and smoldered for a while before catching fire.
Incredibly he bike had a nearly full tank (fiberglass) of petrol which didn't explode. The heart of the fire was on the right hand side just near the back wheel, hence the state of the seat and rear light. I was also surprised to see the alu cases had not melted.
I left the bike with a tarp on it for a month, too demoralized to do anything about it.
Then I though I had to do something, I'd had the bike since new from 1971. I decided to do a repair rather than a restore to get the bike on the road asap.
First thing was to strip off the seat, tank and rear light and other stuff that had melted or burned.
Then gave the whole thing a going over with a degreaser and Karcher to get the soot off. The engine cases were really tarnished by the heat and I ended up using a chunk of wire-wool to get them in a reasonable state.
I then started the parts ordering (good business for Norvil and RGM)

The front tire was ok, just stained with dregs of paint like the rest of the bike as old paint cans exploded with the heat!
The rear had to be changed, I tried a Metzler 18" for a change and easier availability here in France.

Then I gave the tank side panels and front mudguard a good rub down and took them to a local bodywork shop to be painted, cost me 200euros but looks great.
I've been wanting to change the color scheme for years as the metal-flake finish on the tank had become very faded on the top with the sun.



time for breakfast, get back to you later