That red light lets you know when there is a fault in the brake light system (light bulb, wire, switch, whatever). It will stay on until you test your brakes, once the motor is running pull the front brake and step on the rear brake, the light should go off.
If you changed to LED lights, it will stay on unless you also add a resistor. Mine stayed on when I changed to an LED brake light, even with resistors. I eventually pulled the bulb from the cluster and just check the lights by eye more often.
If it ran before, you may have changed something when you last worked on it...maybe? Especially if you changed plugs. Check the wires to make sure they are on the right plugs. It could still have spark but it will be all bassackwards.
Pretty simple machines... good luck.