Don't jerry-rig it!. Do you want to twist the engine around the seatpost into your leg or possibly even fall off the bike?
Mount it down before you try riding it. Make sure all fasteners are tight.
Better safe than sorry.
I wouldn't cut corners breaking it in either. Check fasteners before every ride until you're confident they aren't loosening. Then check them regularly.
Lots of people have successfully utilized the alternator on the engine. Some engines have a more capable alternator than others. At most expect 6V at 3 watts.
It's toward the end of a drawn out project that we are tempted to skip good work habits. We are all guilty of it to some degree or other. But make sure your bike is mechanically sound. You won't regret it later.