I have a snapper walk behind with a fc290v fs04 and it runs for a little while then quits when it gets warm. The resistance on the primary side if the coil with the 5kohm plug attached goes from good, about 17.5k ohms to bad, about 19k ohms. This engine does not have an external igniter. I can't seem to find a replacement 21121 2084 or 20021 2058 coil except on ebay from China for about 100. I was wondering if the 21121-2070 coil from a fc400 would work if I put it on with an external igniter. Has anyone tried this?