thanks for the replies everyone.
unfortunately it's more than a cosmetic issue either way. one of the carb mount drilling has split and almost sheared off back a few inches, top fin on one side long gone, and a few other missing bits of fin too. for the money it would cost to repair - thought i should just buy a replacement cylinder head. and why not get what would work best for the engine - which i still am inclined to believe is alloy. i shall be regrinding the crank, balancing, replacing bushing, seals and bearings, returning the bores to original specs with low compression pistons and hoping that it will serve me well for the next few decades. a big job, but not an unpleasant one for the reward!