Hi Rick,
I put a lot of time into working out the handlebar and stem arrangement on the kit bikes. The Sofrider is pretty well optimized out of the box. My Sofrider is different because it was one of the original Interbike prototypes and had a bar that was slightly too short for the controls to fit on properly, so I replaced the handlebar with the closest thing I could find to the Sofrider part at the time.
The stem that I used on the riser is a 1-1/8" RANS zero-reach stem intended for a crank-forward bike. This moves the bars back about 50mm and allows me to keep the grips close to inline with the steering axis, as you can see in the photo above.
When I have everything right, my knees just miss the cable barrels on the shifters (say, consistent 1/2" clearance). This is one of the advantages of the Silvio layout, where nice. clean brifters are used and the cabling stays clear of the knees altogether.
If I am remembering correctly, you have a Sofrider and are seeing some knee interference. One thing you can do is raise the stem to where the riser tube (I like that term for the part) is about halfway into the stem. This should be adequate to properly retain the stem on the riser, and will give you a little more clearance; this is what I did on the V2 I sent to Bryan Ball for review.
If this doesn't serve, you can get the RANS part here:
http://www.shoprans.com/proddetail.asp?prod=BPHB0049
Another possibility is something like the DNA Saucer:
http://www.billys.co.uk/english/group.php?prod=asdn-saucer&PHPSESSID=3jsjqvmt3v4vh8840a9hdt8225
I don't know what's easily available in Australia...
When you install a stem like these, the existing stemcap simply installs in the end of the riser tube and you adjust the bearing tension prior to installing the stem, which then slips over the top.
Have fun,
Doug