MKII rims are normally fairly easy to find, unless it's a magtek you have, and the 4.5" gives you access to greater tyre choices. I run the BT016's on my R5K and am very happy with them.
Alternatively, if you're not a purist, you can fit the 17" rear from a 21, along with a modified shock linkage to give the extra ride height. Then the choices are unlimted.