Agree:
I would use a proprietary squashed bug remover, something which contains enzymes to break down the goo.
I personally never believed the claims for any paint protecting products. Â£300 will buy a lot of bug remover. However, I was under the impression that the Lifeshine products were guaranteed, whatever that means?

Plenty of softening with warm water and bug remover. I would avoid waxy shampoos as they seal over the problem before it can be removed IE make it worse. Obviously it can't be rushed, but it will come off.

What colour is your motor?