RE: OS120 on 1/4 scale Sig Cub
I have found that mounting four strokes inverted is OK. About the only thing you have to be careful of is that the tank is not placed too high which could cause fuel to syphon into the carb. I would suggest that you use an OS "F" type plug and you should have no problems. I have never needed an on board battery with either my OS 90 FS or my FS 26's on a twin.
Peter