McMaster Carr has some amazing cable for this use even though that is not what it is intended for. I use a vinyl coated cable they offer that is seven strands of seven strands, if that makes sense. It is as flexible as string but is steel. The great part is that you can tie it in a knot instead of crimping. After it is tied you heat the knot with a lighter to just melt the vinyl. The result is a permanent knot. I use it on my 30cc and 50cc planes. Have used it for two seasons and it works great.
McMaster item # 34235T23
FYI, they have fuel line of all types and an amazing amount of items useful to us.