youll need to pull the forks apart to get the size.... brake caliper mounts need to be 52mm center to centre, ive noticed that theres been alot of issues with these avengers....
i think youll find the forks you have are longer then the normal pitbike sizes, the most obtainable pitbike fork length these days are 735mm and 660mm or near abouts, my guess youll have 780mm only a hand ful of pitbikes in the aussie market run them length, such as the more premium brands, itll be just be cheaper to just rebuild them tho