If you get a HK33 kit with cut barrel you are good to go, The 33 cut barrel is long enough to make into a 53 barrel. Or you can get a kit with out the barrel and buy a 53 US made barrel from several resources.
The locking piece from a 33 kit is the same for a 53 (both are 223). This is what I have been told by multiple sources.
The 33 cocking tube can be modified to be used on a 53 (this is the cheapest way to go and hardest if you don't know what your doing) or there are HK parts suppliers who have 53 cocking tubes, and they're not cheap.
I'm working on this same build right now and from my research the parts that need to be modified are:
Bolt carrier - shortened
Recoil spring assy - shortened
Cocking tube support - shortened
Triple frame support coolar - needs to be made and added to barrel.
Here is a link with a 93 to 53 build pics. 9353.htm
You can also talk with gilliebear here on the site he has done some of these builds and is very knowledgeable when it comes to HK convertions.