It reads like you are looking for a spray can clear for a glossy finish.In my experience(which doesnt mean I am right) I have not found a a spray can clear that leaves a glossy you might be looking for.
The spary can clear I have used leaves a flat or matte finish.There is not much of a shine or "depth" to the finish.
I guess the question I have to help in researching a product for you is,how glossy do you want it?
What I can do is contact the paint store I use at work to see what products are avaiable for you to find in your arae that you can use.I will keep my product search to the low but effective end on the price scale.
You are looking for a spray can product or do you have access to clearing this helmet with a paint gun system?
Always happy to help you
Mike