On these old-style engines the location of the mechanical fuel pump (bottom-front of the engine) necessitated the fuel pipes running up and over the distributor to the carburettor, I think that may have had something to do with the fire, particularly if a plastic fuel filter was fitted and sprung a tiny leak, or perhaps the fuel hoses were in the usual degraded condition. I saw a similar result on a FIAT 128 once.
-Alex