If you haven't already, install a working genuine thermostat, and get the fan on auto.
Inspect the ignition system, and make sure it is working perfectly and within specs.
Get the carburettor checked; make sure that the manual choke is connected with the knob on the dashboard. It should idle smoothly at 850-950 RPM, with the mixture properly adjusted and the timing should be 7 deg BTDC.
Make sure that the valve lash is properly adjusted.