Looking for suitable replacement
 

I have a 1987 SR Sport series Cannondale road bike with dia-compe N500 brakes. unfortunately I lost a few pieces of the brakes and was wondering what would be a suitable/best replacement for the brakes or where might I be able to find spare parts to fix it?
I believe the reach 43-59mm

Your best deal right now is a new pair of Cannondale C2 dual pivots. Great brakes that I've used on more than a few late 80's frame-up builds and they will fit nicely.
Just make sure it's not an ST frame (is there an additional bracket above the rear brake bridge?) because those require a longer caliper, especially if converted to 700c from 27".
Wheel & Sprocket has them on eBay for a price so low I hesitate to mention it here.

Actually I checked it again and it is in fact a long reach. Thanks

They didn't fit on my bike