I would cut the old ones off with an angle grinder first, this is pretty easy!
How are you planning to fix the new arches? Will you weld them on? I only ask because that won't be easy, I've done plenty of steel welding, but I wouldn't fancy my chances with aluminium much! Clearly, they don't need to be welded for the MOT and if you decide to stick them somehow then that would be easier. Perhaps you could braze them, maybe someone could help you further with this option?
In all honesty I would try and repair the existing ones, fitting new ones and then getting the line between new and old spot on is not easy at all.