With some horses, you do just have to go larger and cut/add holes/adjust/etc to get a good fit. Our halter body mares don't have 'big' heads, but they do have a very large jaw which affects the way a typical large horse bridle (and halter) fit.
Looks to me like the only problem is the browband, you just need a new peice made that is a little bigger it will replace the browband and you can keep the same bridle. Also, regarding the personalised nylon halter with leather sewn on the sides, I haven't seen them for sale but if you have a nylon halter you could get someone to put the leather overlayed peice on for you. Not sure where you are situated but if you have trouble finding someone let me know, i'm a saddlemaker I can help you with it :).