Ok, what is sounds like is you improperly mixed your resin when you mixed it, or its too cold in the garage to allow a full curing. Since resin cures due a chemical reaction it doesnt air dry. So if its too cold in the garage since you layed the resin, it has been inhibited from curing. Hence the cold is slowing the process down. So instead of hours, its taken months.
So if you compounded the issue by mixing your resin too cold, and then left it in the cold. It could take a very long time to make the smell go away.
Try this,... we use heat lamps at work to increase the temp to a balmy 90 degrees. Provided you have enough air flow, you can recreate this using a space heater, or a shop light.
If the resin did harden, but is still tacky, you didnt mix enough catalyst in it, in which case the only thing you can do is clean off the paint (manually) an then use rubbing alcohol to scrub the enitre thing down to remove any excess uncured resin. If you use any petroluem base solvent to clean it, you wont be able to get any adhesion if you need to repair it, and any future painting wont adhere and leave fish eyes in the paint finish.
Get it cleaned up with rubbing alcohol, and get some heat on it for a few days, maybe even a week and come back to it. it should fix the problem.
If after a week and it is still tacky, or smells, youll have to redo the helmet as it wont be wearable.(fumes)