Is the aluminium body replaceable?

If most dents are on the bottom of the macbook the repair will be quite easy. All that needs done in this case is undo the 6 or so bottom screws and swap the bottom plate. If they are on the side or keyboard/trackpad area they will be a little more tough to fix but still quit doable. In this case you will need to remove the motherboard, battery, track pad, hard drive and all the rest of the cables and hardware that isn't included in the replacement body and reinstall them into the replacement. If they are in the lid then you will probably need a new screen as well because I think the lid is bonded to the lcd. As for replacement parts I would look on the ifixit store or ebay first to see if there are any available. Also if you need instructions to disassemble it they can be found here MacBook Air 13" Early 2015.

I hope all goes well.