Once it's running the idle mix needs to be set much more precisely than you can get away with on an upright engine but the 2-3 minutes that 2MuchThrow mentioned should be more than good enough.
I think thats the key, I've had engines I could ignore the low speed idle mixture screw and run the motor right out of the box with the engine upright. With the engine upside down, you will soon be an expert at the low speed needle value.
As soon as you master getting your engine to run inverted, you will be able to get just about get any motor running in my opinion. So use it as a "learning time"
Most of the people I know give up, and pull an onboard glow...but I think your just "covering" up the problem with that solution.