Look for some Origin8 cranks online. They're cheap, under $50, they look more expensive though. If your bike is newer with some fancy bottom bracket you'll have to buy a traditional square taper BB, shouldn't cost much. You can get them in 155mm length, many people with knee problems find these to be the cure. If you're running a road triple, 130/74 BCD, you should be able to swap your chainrings over.

Truth is, that if you are having knee problems on your diamond frame you're likely to have the same problem on the 'bent. Could be worse as you can push against the back of the seat and really damage your knees. Try the shorter cranks.