Scooterotica

Hi all, looking for a bit of help please, I fitted a pinasco 177 kit to my 1967 Sprint 150, had to remove the electric start ring from the flywheel to get it to fit after trying a dremel to sand a bit off and putting a hole in it ! Now welded back up, rebuilt but was tight, loosened flywheel a bit and would turn over until I fitted the spark plug then wouldn't turn at all, tried a bit harder and sheared the woodruff key, any ideas what I'm missing other than mechanical knowhow, I'm an electrical engineer.