If it has the original honeycomb radiator, they can't really be cleaned effectively once blocked. Hot caustic solutions can be used but the cores can't be "wired out" like a modern core. A simple test is to cap the inlet and outlet, fill radiator with water and remove outlet cap, the radiator should empty itself very quickly, within a few seconds, if it takes longer then you have blocked cores.
Apart from that, timing too retarded, mixture too lean, exhaust restricted.
Good luck!