There are some "how to" s here to remove a bad paint job, like sanding, ovencleaner, etc.
Cracking is fixed with an old t-shirt and super gue, To smooth the crack out some ABS paste is needed.
The you need some cans of plastic primer, and several cans of high quaity.
And as you said, you need replacement parts, like a helmet.
A new, full armor kit will be cheaper