My first thought is:
Why a replacement, known bad by what means?
2nd Why a coil you can not find. Any similar coil that you can find and mount should be fine ( think custom bracket ). Amazon must have a thousand coils you could make work.
Yes drop in replacement is easy peasy, but, maken it work is good enough for me.

Quadra engines (Dick Bennett very knowledgeable):
http://bennettbuilt.com/
http://www.rcuniverse.com/forum/Q35/m_1418707/tm.htm (3/29/04, quadra settings 0.010 coil, 0.015 points (0.013 gives more RPM), 0.025 plug)
Ron Shettler was developer and manufacturer of Quadra engine
Jim Messer was first Quadra dealer
http://www.fly-imaa.org/imaa/hfarticles/engines.html (Engine tips)
http://www.ch-ignitions.com/ (ignition stuff)