It would be very hard to do it in the bike!!!!Is it returning , or slipping on the kick itself?If it's slipping on the kick , all you need is a pawl.( a 1/2 x1/2 x1/4 piece that fits into the end of the kick start.( about $12A) If this fails , the kick will just spin & catch now & then.You must remove the kick start & take the pawl out of the end & replace , then re-tension the kick-start ratchet.If the spring has broken, I don't think that can be replaced with the engine in place either.( I may be wrong & sure the guys with alot more experience may have a way to do it with the engine still in the bike. Which ever , we'll both increase our learning curve lol.Cheers for now , Paul (Brisbane)