I bought a non-running pair of Trail 90’s in 2014 while in Colorado. Resurrected them and sold them off. Had a lot of fun, but with tired engines and 8,000 foot elevations they did NOT make the full 8 horsepower! Ha!Probably your battery....as I recall, the battery serves as a sort of voltage regulator, and is an important part of the ignition circuit.
My 1974 model was a 6 volt....not 12.