I'm using a 28. I don't have much experience with smaller engines, but the plane is pretty light, so i suspect it will fly with a 20, but they all fly better with more engine. Give it a shot, just keep it light, (350 mAh battery, mini servo on throtle...) and don't do the cowl untill you are sure you like the engine.

Low speed flight? why would you want to do that?! it lands slow if thats what you are asking. Never seen this plane tip stall, it tracks nice. Good inverted performance.

Getting into the air can be a pain if the wheels aren't perfectly alligned, it might be worth the effort to engineer a tail wheel (comes with a skid)