I have been flying a Scratch built Ryan's Rebel for about 3 years now with a home converted Ryobi 31cc engine. It has always flown very well, turning about 7300 to 7500 on a Zinger 16 by 6-10 wood prop. About 4 months ago I built a Timm-6 ignition using one of Jerry Howells kits and coil. I cut the center out of an old Ryobi flywheel and ended up with about a 3/4 inch diameter piece. I drilled two holes as close together as I could and placed in each a 1/8" rare earth magnet both of which I got from Radio Shack. That provided enough Dwell time for the coil to charge between sparks. I put this system on My Ryobi on the Rebel after a number of bench runs. I have probably flown it 25 to 30 times since, and have been very pleased with it. It still turns the same RPM as before, but is now some lighter. I am using a 5 cell 1800 mah NIMH battery pack on the TIMM 6 and have not had to recharge it at the field. It holds up as good as does the 4 cell NIMH 1800mah flight pack battery.
I have also built one of the CDI units designed by Floyd E. Carter that was published in a 1980 RCM issue. I have only bench ran it on a Ryobi and it seems to work well, but I have not yet flown it. At the time I built it I could not find the specified toroid coil, so I wound the transformer on an old transistor radio antenna ferrite rod about 1 1/2 inches long and it worked fine. It may produce a little more RFI than would the toroid, but it does not appear to be a problem.
I do enjoy this hobby!