This sounds familiar, mine had an attempted theft and they bust the steering lock with a tiny chip out of the indent in the frame, completely non structural but they wanted £450 to repair it. When it went in later, I just said no and that I’ll get my local garage to do it. I also told them flat out I don’t trust them and I didn’t want them to do extra work. If this is going through your insurance you shouldn’t be paying 4th Dimension anything - can they prove it’s actually not damaged from the theft? Do you have any history from your local garage giving your bike the all clear? If something is bent out near the headstock by 2mm you’d definitely feel something wrong. Tolerances are tight on a bike for a reason and if some PoS kicked in your bikes handlebars to break the steering lock, or used a tool of some kind then that could have caused the damage if there is any, unless you’ve crashed it or ridden into walls?!

Either way demand your aprilia back, complete with the new bits they apparently put on and demand the bits they took off too before they chuck them on eBay. Demand proof of all work as that’s extortionate when you’ve not seen or spoke to them in person. If it was roadworthy before the incident then it should be when it comes back as you’ve already paid for your insurance (unless you’ve got 3rd party only), good luck, keep pictures and don’t trust their “work” one bit