The Ultramag was never available with spirals, so the Hoyt tune charts do not list any string/cable lengths for the combination.Changing the cam & 1/2 cams will need new string/cables as the lengths will be different.
If you are going to do the job yourself, the best place to get parts is from the
ArcheryTalk forum in the states and get the cams second hand. (About £45 at the moment with the week dollar!) And then some strings from a guy called Bucknasty on the same forum ($52, about £27).
If you are going to get a shop to do the change, it may be better to have a word with them to see that they are OK with you sourcing all the parts, then get them from the states!
It is fairly radical surgery for a bow, but actually quite easy to do if you are careful.
Alex.