I doubt more than an ounce or two of oil would cling to the innards of a tank (if it's drained completely). Rinse the tank with some gasoline and save it.. add it to your next few batches.
If the tank is already refilled, just run it.. Too much oil in the fuel mix is not a problem compared to too little. Too much and you get some smoke.. too little and you toast the engine.
Invest in a Kreem kit.. motorcycle shop.