You converted to a generator?
Have you got a temperature gauge connected? If not connect one, as you can get what looks like steam from the coolant even though it is not boiling.
2 minutes is very quick for it to overheat from cold - unless you mean 2 minutes of driving after having warmed it up. I would look for any blockages or perhaps a dud thermostat (is the top radiator hose hot, and have coolant in it?)
I would also recommend you check the timing and tune the carburettor properly - a mixture of incorrect ignition timing and lean carburettor can make the engine run artificially hot and cause all sorts of problems.