The IHP kit is a pretty close match for an early 50`s Sydney, but there are a number of changes to be made.
Off the top of my head you would need to change the position of the openings either side of the ship (situated just in front of the forward boat storage on the model). They will need to be filled and new ones cut between the forward gun sponsons.
The gun director sponsons are different, with higher sides, and later twin 40mm Bofors Mark V (I think) were used for main air defence.
It would be easier to use IHP`s Theseus or Triumph kit as these have the later closed bridge used on Sydney. The island would then have to be further modified for Sydney`s fit.
You should be able to use/modify the stern catwalk from that found on the Toms PE set to match Sydney.
Thatís all the major mods I can think of. Hope this helps.