I have a 20HP onan opposed twin sitting in my shed- cranks but has no spark.
You can get parts at specific tractor parts places - possibly RV repair places as well ( most RV's use onan motors ) that carry onan parts .
Be prepared to dish out alot of cash - parts arent cheep.
Ill have to look into a harely coil - theres a couple motorcycle places nearby- thats the problem with twins, need it set up for 2 wires.