yes it will mate. will be a 710 or 714....f7r 710/714, williams lump being f7r 700
f7r 700/710 virtually identical, 714 has a chain driven oil pump rather than the belt driven one on the 700/710.
valver head off an 1800 will bolt straight on to any of the 3, but bigger valves and better porting on the williams head. the meg head has bigger valves again.