Restoring/Refinishing a fiberglass tub, boat, car-part, whatever, may be best left to a Pro. I've never seen it, nor have I ever seen it done. With that, perhaps others can offer some advice for the DIYer or otherwise.
Here's an article about Fiberglass Tub Cleaning and Fixing with some contact info that may be able to guide you to DIY Refinishing/Restoration. Call the leads in the article for more info.
My best to ya and hope this helps.
Jay J -Moderator
PS: Be sure you read any Warranty info BEFORE you buy. My 'gut' tells me the refinishing won't be as good as new or Warrantied like new ...